Ruby MatchData Documentation Revised

I've revised the official Ruby documentation for class MatchData, which is powerful, but (I think) underutilized and often not well-understood.

• Made examples more consistent.
• Added much detail.
• Emphasized captures.
• Added section "What's Here" (bullet lists of methods and their effects).

Check it out:

• Revised (to be in release 4.1, next month): https://docs.ruby-lang.org/en/master/MatchData.html.
• Existing (release 4.0): https://docs.ruby-lang.org/en/4.0/MatchData.html.