docs(block): add changeset for overflow bug

Add a changeset that explains that overflowing blocks no longer
create an empty line in block diagrams or cause bugs in later lines.

Technically, this is a breaking change since it can change the layout of
block diagrams, but this diagram is still in `-beta` and they all will
still render correctly.

.changeset/lovely-queens-own.md

@@ -0,0 +1,8 @@
+---
+'mermaid': patch
+---
+
+fix(block): overflowing blocks no longer affect later lines
+
+This may change the layout of block diagrams that have overflowing lines
+(i.e. block diagrams that use up more columns that the `columns` specifier).