crecord: update uiheader docstring

There's no need to move anything to patch.py. The uiheader class only
has methods relevant to crecord and overrides __getattr__ in order to
use `patch.header` objects as a sort of mixin.

mercurial/crecord.py

@@ -201,9 +201,8 b' class patch(patchnode, list): # todo: r'
 
 
 class uiheader(patchnode):
-    """patch header
-
-    xxx shouldn't we move this to mercurial/patch.py ?
+    """
+    patchnode class wrapping a patch.header
     """
 
     def __init__(self, header):