On the log-addressing branch: Follow-up to r1510990, documenting
another in-txn file used by format7.

* subversion/libsvn_fs_fs/structure
  (Transaction layout): mention the "itemidx" file

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1511382 13f79535-47bb-0310-9956-ffa450edef68