Moved "`mod_expires` to be enabled" to new line.

- Right now the comment was not rendering properly.
  # The Expires* directives requires the Apache module
  `mod_expires` to be
  # enabled.
- After this change -
  # The Expires* directives requires the Apache module
  # `mod_expires` to be enabled.
Removing merge conflict remnants in the guides.
Document ability to run a single test.
Fix doc for singularize - `pluralized` => `singularized`
Note functional equivalence in reset task
Add documentation for rake db:setup task
Fix typos [ci skip]
Merge branch 'master' of github.com:rails/docrails