Document XY chart legend release notes

Follow Mermaid contribution guidance by marking the new docs section with the release token and adding a changeset.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: xdumaine

.changeset/xychart-legends.md

@@ -0,0 +1,5 @@
+---
+'mermaid': minor
+---
+
+feat: add legends for named XY chart line and bar series

docs/syntax/xyChart.md

@@ -98,7 +98,7 @@ A bar chart offers the capability to graphically depict bars.
 1. `bar [2.3, 45, .98, -3.4]` it can have all valid numeric values.
 2. `bar "series name" [2.3, 45, .98, -3.4]` adds the bar to the legend.
 
-### Legend
+### Legend (v\<MERMAID_RELEASE_VERSION>+)
 
 Named line and bar plots are automatically shown in a legend. Unnamed plots are omitted from the legend.
 

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

@@ -86,7 +86,7 @@ A bar chart offers the capability to graphically depict bars.
 1. `bar [2.3, 45, .98, -3.4]` it can have all valid numeric values.
 2. `bar "series name" [2.3, 45, .98, -3.4]` adds the bar to the legend.
 
-### Legend
+### Legend (v<MERMAID_RELEASE_VERSION>+)
 
 Named line and bar plots are automatically shown in a legend. Unnamed plots are omitted from the legend.