docs: document new syntax for half-arrow message and central connection

on-behalf-of: @Mermaid-Chart <hello@mermaidchart.com>

docs/syntax/sequenceDiagram.md

@@ -209,7 +209,11 @@ Messages can be of two displayed either solid or with a dotted line.
 [Actor][Arrow][Actor]:Message text
 ```
 
-There are ten types of arrows currently supported:
+Lines can be solid or dotted, and can end with various types of arrowheads, crosses, or open arrows.
+
+#### Supported Arrow Types
+
+**Standard Arrow Types**
 
 | Type     | Description                                          |
 | -------- | ---------------------------------------------------- |
@@ -224,6 +228,76 @@ There are ten types of arrows currently supported:
 | `-)`     | Solid line with an open arrow at the end (async)     |
 | `--)`    | Dotted line with a open arrow at the end (async)     |
 
+**New Half-Arrows**
+
+The following half-arrow types are now supported for more expressive sequence diagrams. Both solid and dotted variants are available by increasing the number of dashes (`-` → `--`).
+
+---
+
+\| Type | Description |
+\| ------- | ---------------------------------------------------- | ---------------------------------------------- |
+\| `-\|\` | Solid line with top half arrowhead |
+\| `--\|\` | Dotted line with top half arrowhead |
+\| `-\|/` | Solid line with bottom half arrowhead |
+\| `--\|/` | Dotted line with bottom half arrowhead |
+\| `/\|-` | Solid line with reverse top half arrowhead |
+\| `/\|--` | Dotted line with reverse top half arrowhead |
+\| `\\     | -` | Solid line with reverse bottom half arrowhead |
+\| `\\     | --` | Dotted line with reverse bottom half arrowhead |
+\| `-\\` | Solid line with top stick half arrowhead |
+\| `--\\` | Dotted line with top stick half arrowhead |
+\| `-//` | Solid line with bottom stick half arrowhead |
+\| `--//` | Dotted line with bottom stick half arrowhead |
+\| `//-` | Solid line with reverse top stick half arrowhead |
+\| `//--` | Dotted line with reverse top stick half arrowhead |
+\| `\\-` | Solid line with reverse bottom stick half arrowhead |
+\| `\\--` | Dotted line with reverse bottom stick half arrowhead |
+
+## Central Connections
+
+Mermaid sequence diagrams now support **central lifeline connections** using a `()`.\
+This is useful to represent messages or signals that connect to a central point, rather than from one actor directly to another.
+
+To indicate a central connection, append `()` to the arrow syntax.
+
+#### Basic Syntax
+
+```mermaid-example
+sequenceDiagram
+    participant Alice
+    participant John
+    Alice->>()John: Hello John
+    Alice()->>John: How are you?
+    John()->>()Alice: Great!
+```
+
+```mermaid
+sequenceDiagram
+    participant Alice
+    participant John
+    Alice->>()John: Hello John
+    Alice()->>John: How are you?
+    John()->>()Alice: Great!
+```
+
+```mermaid-example
+sequenceDiagram
+    participant Alice
+    participant John
+    Alice->>()John: Hello John
+    Alice()->>John: How are you?
+    John()->>()Alice: Great!
+```
+
+```mermaid
+sequenceDiagram
+    participant Alice
+    participant John
+    Alice->>()John: Hello John
+    Alice()->>John: How are you?
+    John()->>()Alice: Great!
+```
+
 ## Activations
 
 It is possible to activate and deactivate an actor. (de)activation can be dedicated declarations:

packages/mermaid/src/docs/syntax/sequenceDiagram.md

@@ -144,7 +144,11 @@ Messages can be of two displayed either solid or with a dotted line.
 [Actor][Arrow][Actor]:Message text
 ```
 
-There are ten types of arrows currently supported:
+Lines can be solid or dotted, and can end with various types of arrowheads, crosses, or open arrows.
+
+#### Supported Arrow Types
+
+**Standard Arrow Types**
 
 | Type     | Description                                          |
 | -------- | ---------------------------------------------------- |
@@ -159,6 +163,58 @@ There are ten types of arrows currently supported:
 | `-)`     | Solid line with an open arrow at the end (async)     |
 | `--)`    | Dotted line with a open arrow at the end (async)     |
 
+**New Half-Arrows**
+
+The following half-arrow types are now supported for more expressive sequence diagrams. Both solid and dotted variants are available by increasing the number of dashes (`-` → `--`).
+
+---
+
+| Type    | Description                                          |
+| ------- | ---------------------------------------------------- | ---------------------------------------------- |
+| `-\|\`  | Solid line with top half arrowhead                   |
+| `--\|\` | Dotted line with top half arrowhead                  |
+| `-\|/`  | Solid line with bottom half arrowhead                |
+| `--\|/` | Dotted line with bottom half arrowhead               |
+| `/\|-`  | Solid line with reverse top half arrowhead           |
+| `/\|--` | Dotted line with reverse top half arrowhead          |
+| `\\     | -`                                                   | Solid line with reverse bottom half arrowhead  |
+| `\\     | --`                                                  | Dotted line with reverse bottom half arrowhead |
+| `-\\`   | Solid line with top stick half arrowhead             |
+| `--\\`  | Dotted line with top stick half arrowhead            |
+| `-//`   | Solid line with bottom stick half arrowhead          |
+| `--//`  | Dotted line with bottom stick half arrowhead         |
+| `//-`   | Solid line with reverse top stick half arrowhead     |
+| `//--`  | Dotted line with reverse top stick half arrowhead    |
+| `\\-`   | Solid line with reverse bottom stick half arrowhead  |
+| `\\--`  | Dotted line with reverse bottom stick half arrowhead |
+
+## Central Connections
+
+Mermaid sequence diagrams now support **central lifeline connections** using a `()`.  
+This is useful to represent messages or signals that connect to a central point, rather than from one actor directly to another.
+
+To indicate a central connection, append `()` to the arrow syntax.
+
+#### Basic Syntax
+
+```mermaid-example
+sequenceDiagram
+    participant Alice
+    participant John
+    Alice->>()John: Hello John
+    Alice()->>John: How are you?
+    John()->>()Alice: Great!
+```
+
+```mermaid
+sequenceDiagram
+    participant Alice
+    participant John
+    Alice->>()John: Hello John
+    Alice()->>John: How are you?
+    John()->>()Alice: Great!
+```
+
 ## Activations
 
 It is possible to activate and deactivate an actor. (de)activation can be dedicated declarations: