Add gcloud.* and gcloud.datastore.* docstrings to pacify pylint.

See https://github.com/GoogleCloudPlatform/gcloud-python/pull/238#issuecomment-58954981.
PEP8 mandates that docstrings wrap at 72 characters.  I prefer to always wrap the closing quotes, in order to avoid multi-line diffs when editing causes them to expand / contract around that boundary.  PEP 257 does show short single-line docstrings with the trailing quotes on the same line.