add explicit extension disabling syntax to hgrc.5

doc/hgrc.5.txt

@@ -216,6 +216,10 b' extensions::'
   the path to the ".py" file (including the file name extension) that
   defines the extension.
 
+  To explicitly disable an extension that is enabled in an hgrc of
+  broader scope, prepend its path with '!', as in 
+  'hgext.foo = !/ext/path' or 'hgext.foo = !' when no path is supplied.
+
   Example for ~/.hgrc:
 
     [extensions]