Fix formatting for feature description.

RelishApp and Cucumber are very picky about syntax. **ANY** line which
starts with an octothorpe is treated as a comment. This makes the lexer
very unhappy.

In order to trick it I had to add a unicode space (specifically the zero
width space <200b>). This alone didn't fix things. For some reason,
using a semi-colon fixed things. I tried several additional variations,
all of which worked; but they each still required the semi-colon:

- `controller { ; }`
- `controller do; end`

I settled on this various as I felt it read a bit better with the
comment being in the `controller` definition.
Merge pull request #1040 from rspec/controller-doc-fixes

Fix formatting for feature description.
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.