Follow-up to r1580406.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__get_file_delta_stream): Update commentary. No functional change.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1580629 13f79535-47bb-0310-9956-ffa450edef68