Follow-up to r1686541: document the confusing interface.
Follow-up to r1686541: document the confusing interface.

* subversion/tests/cmdline/svntest/main.py
  (chmod_tree): Document.

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1686867 13f79535-47bb-0310-9956-ffa450edef68

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