The structure of FSX is nowhere near its completion.

So, remove everything *not* currently applying to FSX from the
'structure' file carried over from FSFS, put a disclaimer at
the top and mark the mostly empty sections as TBD.

* subversion/libsvn_fs_x/structure
  (): add disclaimer
  (Design,
   Filesystem formats,
   Revision file format): put a "TDB" in here and remove most text
  (Packing revision*): drop entirely
  (Layout of the FS directory,
   Node-revision IDs,
   Transaction layout): update to reflect the current state of FSX
  (Locks layout): s/FSFS/FSX/


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1510158 13f79535-47bb-0310-9956-ffa450edef68