FSFS Documentation update.  No functional change.

* subversion/libsvn_fs_fs/structure-indexes
  (): The final index data is no longer stored in separate files.

* subversion/libsvn_fs_fs/structure
  (): Same.
  (Revision file format): Describe the new f7 revision file footer.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1621275 13f79535-47bb-0310-9956-ffa450edef68