Hi, here's my small contribution to the documentation in relation to #819
Few things:
* the `delete_release` method always returns `True` so not sure if the return type is truly valid - potentially a bug
* I've tried generating the documentation locally to check if formatting is ok but failed - is there a guide to it? I followed the read the docs starter guide but that didn't seem to reflect the way it's done here
* Added is_draft and is_prerelease property to GitRelease
* Added target_commitish option to Repository.create_git_release()
* Added test for Repository.create_git_release()
* Renamed attr of GitRelease: is_draft, is_prerelease -> draft, prerelease