Cleanup `all` matcher cuke a bit.

- It doesn't use `all?`; it just needs an be enumerable.
- We don't need so many failed examples, and they certainly
  don't need comments.
- For compound matchers it reads best to use the base
  matcher (e.g. `start_with`) rather than an alias
  (`starting_with`).
Cleanup changelog entry.

- It had an example of the negated matcher operator
  that Adam was working on in another PR. It
  doesn't belong here.
- Keep the changelog entry simple: just show a
  simple example, not a compound example.
Cleanup `all` YARD docs.

- No, it doesn't work for strings (and I don't see a point
  in it for a string).
- We specifically don't support `not_to all`, so we shouldn't
  include an example of that.
- Clarify notes.
Merge pull request #528 from rspec/all-matcher-improvements

All matcher improvements