Kitchen sink tweaks

Subheader presentation plus <dt>, <dl> and <dd> fixes.
Kitchen sink tweaks

Better describe dropdown button types
Kitchen sink tweaks

Tighten wording on panels.
Kitchen sink tweaks

Tighten wording on reveal.
Kitchen sink tweaks

Expanded labels on split buttons
Merge pull request #5323 from petecooper/master

Kitchen sink doc tweaks - no new functionality