Fix docs for DS retries

[rob05c]

Adds and clarifies docs for simple and unavailable retry parameters. Also fixes some related code comments.

Which Traffic Control components are affected by this PR?

• Documentation

What is the best way to verify this PR?

Generate docs, observe they match code and ATS behavior.

If this is a bugfix, which Traffic Control versions contained the bug?

Not a bug fix.

PR submission checklist

• ~[x] This PR has tests~ no tests, no code change
• [x] This PR has documentation
• ~[x] This PR has a CHANGELOG.md entry~ no changelog, docs fix is too trivial to clutter the changelog
• [x] This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)