dirstate: state that getcwd() shouldn't be used to get real file path

hgweb will force it to be '' so that file patterns can be resolved relative
to the repository root. I want to clarify that is correct.

mercurial/dirstate.py

@@ -220,6 +220,12 b' class dirstate(object):'
         return os.getcwd()
 
     def getcwd(self):
+        '''Return the path from which a canonical path is calculated.
+
+        This path should be used to resolve file patterns or to convert
+        canonical paths back to file paths for display. It shouldn't be
+        used to get real file paths. Use vfs functions instead.
+        '''
         cwd = self._cwd
         if cwd == self._root:
             return ''