Improve docs based on feedback from @xaviershay.
Pare down the number of matcher aliases.

- The `RSpec::Matchers` API was getting very large with
  all the aliases, which makes it harder to keep all in
  your head.
- It's better to start with fewer and then add more
  in future releases as there is demand. Once they are
  there, we can't remove them from a future minor/patch
  release because of server.
- `RSpec::Matchers.alias_matcher` is provided so users can
  define new aliases.
Remove method_missing support for dynamic predicate matcher aliases.

It was going to be very hard to document how this worked,
and what phrasing forms are supported. Instead, we simply allow
users to define their own aliases using `alias_matcher`.