Document which API endpoints can return a 503 response (BB-3788)

If a repository is in the creation process, or is temporarily unavailable, the API can return 503 responses, however, we don't document which endpoints may do so.

This makes it harder for developers to robustly use our API.

For example, for a repository that was in the creating state,

• {{ {https://api.bitbucket.org/1.0/repositories/:username/:reposlug/events}

  }} returns 503,

• {{ {https://api.bitbucket.org/1.0/repositories/:username/:reposlug}

  }} returns info about the repository.