Add repo_mapping to documentation

### Description of the problem / feature request:

An important attribute, repo_mapping, is not listed in the documentation.

https://docs.bazel.build/versions/master/repo/http.html

I only found out about this through a github issue:
https://github.com/bazelbuild/bazel/issues/3219

This should really be listed in the repository documentation for http_archive. It would have saved me a huge headache, and prevented issues created like the one listed above.