Doc coverage.

* Many things are private.
* Added docs tend to be terrible, I don't have enough context to flesh
  them out.
* Still some warnings in here I don't know what to do about.
* What do we do about CI? Can rspec-dev add it for us?
Documentation for configuration.
Unbreak MailerExampleGroup loading.
Better documentation for #with
Backticks
Documentation for routes=

@cupakromer not really sure if this is what you were asking for in
https://github.com/rspec/rspec-rails/pull/1034#discussion_r12906450
Better wording for RSpec::Rails::ViewAssigns.
Check doc coverage in travis build.

This is similar but different (see the --exclude) to the function
provided in rspec-dev, so not providing a way to sync them. It's a
simple enough function anyways.
Need to include documentation in travis bundle for YARD checks.
Fix broken .yardopts file
Merge pull request #1034 from rspec/doc-coverage

Doc coverage