Improvements in the REST API documentation index page

- Reword the "response codes" section and add a link to the
  relevant HTTP specification.

- Consistently say "is returned" instead of "is used" in response
  descriptions.

- Improve wording of "403 forbidden" response description.

- Reword the authentication section and mention that HTTP basic
  authentication can be used by setting auth.gitBasicAuth.

Change-Id: Id25034267cef668a7d9af82cbdacce744a0e5243