Clarifies what `GET /user/repos` returns.
There have been several instances of people contacting support,
confused about the why `GET /user/repos` doesn't include repos
owned by organizations which the user can access. This clarifies
what's returned.
Also link to `GET /orgs/:org/repos` docs.
Merge pull request #293 from github/jdennes-clarify-list-your-repositories

Clarifies what `GET /user/repos` returns.