bookmarks: document getbkfile method

I'm working on bmstore again, and this function gave me a moment's
pause. Document it to save future readers from any undue confusion.

mercurial/bookmarks.py

@@ -61,6 +61,12 b' class bmstore(dict):'
                 raise
 
     def getbkfile(self, repo):
+        """Hook so that extensions that mess with the store can hook bm storage.
+
+        For core, this just handles wether we should see pending
+        bookmarks or the committed ones. Other extensions (like share)
+        may need to tweak this behavior further.
+        """
         bkfile = None
         if 'HG_PENDING' in os.environ:
             try: