Add docs for custom glyphs changes

[Tyriar]

microsoft/vscode#284758

[DakotaB75]

This makes sense to document.

It may be helpful to explicitly call out:

• when the custom glyph behavior changed (release/version)
• whether the change affects existing themes or only newly authored ones
• a short before/after example to make the impact obvious for theme authors

Framing this as a non-breaking vs potentially breaking change would also help
set expectations and reduce upgrade confusion.