dirs: document Py_SIZE weirdness

dirs: document Py_SIZE weirdness

Assigning to what looks like a function is clown shoes. Document that
it is a macro referring to a struct member.

--- a/mercurial/dirs.c	Wed Oct 12 12:22:54 2016 +0200
+++ b/mercurial/dirs.c	Sat Oct 08 17:07:43 2016 +0200
@@ -76,6 +76,8 @@
 			if (key == NULL)
 				goto bail;
 		}
+		/* Py_SIZE(o) refers to the ob_size member of the struct. Yes,
+		* assigning to what looks like a function seems wrong. */
 		Py_SIZE(key) = pos;
 		((PyBytesObject *)key)->ob_sval[pos] = '\0';