docs(button): update spacing note

[TomJGooding]

Update the note in the Button docs about the spacing, as the default CSS for this widget was changed in #3539.

Closes #3793.

Please review the following checklist.

• [ ] Docstrings on all new or modified functions / classes
• [x] Updated documentation
• [ ] Updated CHANGELOG.md (where appropriate)

[TomJGooding]

There's also no mention of pseudo classes which is likely to catch some people out. Moved into draft until I find a concise way to explain this.