Move formatter protocol docs to a psuedo class.

As discussed in #1627, similar to how @myronmarston improved the matcher protocol docs, this moves the formatter protocol documentation out of the base formatter and into a psuedo class.
s/regster/register/

Also, I think it's best to use the fully qualified constant name since 3rd party formatters should not be namespaced within `RSpec::Core`, and thus this wouldn't work.