Docstring updates. No functional change.

* subversion/libsvn_fs_fs/low_level.h
  (): Mention revision footer.
  (svn_fs_fs__change_receiver_t): Add docstring.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605059 13f79535-47bb-0310-9956-ffa450edef68