docs/Releases: update process documentation.

- Note that Homebrew/homebrew-core CI jobs should complete successfully
  before creating a release
- No code changes only required for 4 hours, 24 hours is a bit
  excessive/unrealistic
- Note that commented `odeprecated` code should be uncommented
- TinyLetter emails are no longer required (MailChimp does so
  automatically from the RSS feed)
- Note that `brew release` will output the major/minor blog post content
  (after #10499 is merged) instead of referencing the now-deprecated
  `brew release-notes`
- Note why you should not create a release on older `master` commits and
  provide an alternate process to handle this case
- Various style tweaks