More issue #745 work.  Now, I think I'm finished.

* subversion/include/svn_fs.h
  (svn_fs_path_change_kind_t): Document values.
  (svn_fs_path_change_t): Document members.

* subversion/libsvn_fs/tree.c
  (undelete_change): New helper function for undoing a deletion change.
  (merge): Remove the "delete" change whenever this transaction wasn't
    actually responsible for it (it was a safe merge from a prior
    commit).  Also, update docstring outline to reflect what the code is
    actually doing in some places.

* subversion/libsvn_fs/bdb/changes-table.c
  (svn_fs__open_changes_table): Remove prophetic pseudo-code.
  (make_change): Change docstring to reflect the Truth.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/issue-745-dev@842560 13f79535-47bb-0310-9956-ffa450edef68