Update FSFS documentation.  No functional change.

* subversion/libsvn_fs_fs/structure
  (Revision file format): Document that the sloppy "ommit expanded size"
                          practice is allowed for property reps only.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1561421 13f79535-47bb-0310-9956-ffa450edef68