chgserver: document why we don't merge mtimehash and confighash

People may get confused about chg's mtimehash and confighash design: why two
hashes instead of just one. This patch adds text addressing the concern.

hgext/chgserver.py

@@ -154,6 +154,10 b' def _mtimehash(paths):'
 
     for chgserver, it is designed that once mtimehash changes, the server is
     considered outdated immediately and should no longer provide service.
+
+    mtimehash is not included in confighash because we only know the paths of
+    extensions after importing them (there is imp.find_module but that faces
+    race conditions). We need to calculate confighash without importing.
     """
     def trystat(path):
         try: