bpo-37206: Unrepresentable default values no longer represented as None. (GH-13933)

In ArgumentClinic, value "NULL" should now be used only for unrepresentable default values
(like in the optional third parameter of getattr). "None" should be used if None is accepted
as argument and passing None has the same effect as not passing the argument at all.
diff --git a/Objects/clinic/dictobject.c.h b/Objects/clinic/dictobject.c.h
index b87244d..7395e3b 100644
--- a/Objects/clinic/dictobject.c.h
+++ b/Objects/clinic/dictobject.c.h
@@ -117,10 +117,10 @@
 }
 
 PyDoc_STRVAR(dict_pop__doc__,
-"pop($self, key, default=None, /)\n"
+"pop($self, key, default=<unrepresentable>, /)\n"
 "--\n"
 "\n"
-"Remove specified key and return the corresponding value.\n"
+"D.pop(k[,d]) -> v, remove specified key and return the corresponding value.\n"
 "\n"
 "If key is not found, default is returned if given, otherwise KeyError is raised");
 
@@ -190,4 +190,4 @@
 {
     return dict___reversed___impl(self);
 }
-/*[clinic end generated code: output=0fd5cafc61a51d3c input=a9049054013a1b77]*/
+/*[clinic end generated code: output=4d98145508da8fa3 input=a9049054013a1b77]*/