doc(er): add documentation on multiple key constraints

docs/syntax/entityRelationshipDiagram.md

@@ -200,14 +200,13 @@ The `type` and `name` values must begin with an alphabetic character and may con
 
 #### Attribute Keys and Comments
 
-Attributes may also have a `key` or comment defined. Keys can be "PK", "FK" or "UK", for Primary Key, Foreign Key or Unique Key. And a `comment` is defined by double quotes at the end of an attribute. Comments themselves cannot have double-quote characters in them.
+Attributes may also have a `key` or comment defined. Keys can be `PK`, `FK` or `UK`, for Primary Key, Foreign Key or Unique Key. To specify multiple key constraints on a single attribute, separate them with a comma (e.g., `PK,FK`).. A `comment` is defined by double quotes at the end of an attribute. Comments themselves cannot have double-quote characters in them.
 
 ```mermaid-example
 erDiagram
     CAR ||--o{ NAMED-DRIVER : allows
     CAR {
-        string allowedDriver FK "The license of the allowed driver"
-        string registrationNumber UK
+        string registrationNumber PK
         string make
         string model
         string[] parts
@@ -217,17 +216,21 @@ erDiagram
         string driversLicense PK "The license #"
         string(99) firstName "Only 99 characters are allowed"
         string lastName
+        string phone UK
         int age
     }
-    MANUFACTURER only one to zero or more CAR
+    NAMED-DRIVER {
+        string carRegistrationNumber PK,FK
+        string driverLicence PK,FK
+    }
+    MANUFACTURER only one to zero or more CAR : makes
 ```
 
 ```mermaid
 erDiagram
     CAR ||--o{ NAMED-DRIVER : allows
     CAR {
-        string allowedDriver FK "The license of the allowed driver"
-        string registrationNumber UK
+        string registrationNumber PK
         string make
         string model
         string[] parts
@@ -237,9 +240,14 @@ erDiagram
         string driversLicense PK "The license #"
         string(99) firstName "Only 99 characters are allowed"
         string lastName
+        string phone UK
         int age
     }
-    MANUFACTURER only one to zero or more CAR
+    NAMED-DRIVER {
+        string carRegistrationNumber PK,FK
+        string driverLicence PK,FK
+    }
+    MANUFACTURER only one to zero or more CAR : makes
 ```
 
 ### Other Things