[ABI] Document associated conformance descriptors.

Author: DougGregor

docs/ABI/Mangling.rst

@@ -141,7 +141,7 @@ types where the metadata itself has unknown layout.)
   global ::= protocol 'TL'               // protocol requirements base descriptor
   global ::= assoc-type-name 'Tl'        // associated type descriptor
   global ::= assoc-type-name 'TM'        // default associated type witness accessor
-
+  global ::= type assoc-type-path protocol  // associated conformance descriptor
 
   REABSTRACT-THUNK-TYPE ::= 'R'          // reabstraction thunk helper function
   REABSTRACT-THUNK-TYPE ::= 'r'          // reabstraction thunk