Refactor example group macros.

They were structured so that they were defining a singleton method on the singleton class (meaning it actually got defined
on the singleton class of the singleton class) which makes
my head hurt. This is far simpler.
Remove unnecessary example group methods.

We still need a 2.99 deprecation for these.
Improve docs.
Merge pull request #1411 from rspec/code-audit-changes-part-3

Code audit changes part 3