Document the packed revprop structures of FSFS format 6.
Document the packed revprop structures of FSFS format 6.

* subversion/libsvn_fs_fs/structure:
  (Layout of the FS directory): add packed revprop shards
  (Filesystem formats): add format 6
  (Packing revision properties (format 6+)): new section

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1358633 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion