fix docstrings to conform to reStructuredText format

[dandhlee]

Currently there are some files like https://github.com/googleapis/python-spanner/blob/master/google/cloud/spanner_v1/session.py which has docstrings that do not conform to Google Docstrings -> reStructuredText format.

For example:

:raises: :exc:`ValueError` if :attr:`session_id` is already set.

should be

:raises ValueError: if :attr:`session_id` is already set.

and

:raises: ValueError: if :attr:`session_id` is not already set.

should be

:raises ValueError: if :attr:`session_id` is not already set.

Please close this issue once there aren't any more of these malformed docstrings in this repository.