Change lib/set.rb from markdown to rdoc

[BurdetteLamar]

Proposing to change RDoc format in lib/set.tb from markup: markdown to default rdoc.

My reason is this:

• I've written a section "What's Here" that's a collection of categorized and linked 1-line descriptions of the methods. I've done this already for classes Hash, Array, and String.
• I've mistakenly put the new section into a PR over at ruby/ruby, and have since learned that it should be done here.
• However, it turns out that the definition list format I've used has big problems in the markdown format that's used here:
  • No DL items are properly recognized if the "term" is a non-word or especially if it's a link.
  • A non-auto link (e.g., {^}[#method-i-5E] sometimes works, but is rendered with no change of bgcolor or font, and so is not obviously a link.

So this PR proposes to change from markdown to rdoc, where these DLs and links are well-behaved.

I think I've handled the formatting throughout correctly, but if there are errors I'll be happy to fix them.

If this PR is approved and merged, I'll bring over the actual section "What's Here" in another PR.

I'm requesting a review by @knu, and hope that @jeremyevans will also take a look.

[BurdetteLamar]

@knu, if you're not interested in this, please let me know. I'll close this PR and move on. Thanks.

[BurdetteLamar]

I have adapted the addition to markdown in a new PR.