SF bug 630824: pydoc Helper keywords missing 'yield'

Wow, what a brittle subsystem!  Fixed, maybe, provided Fred doesn't
shuffle the docs around.

Bugfix candidate.

Lib/pydoc.py

@@ -1406,6 +1406,7 @@ class Helper:
         'return': ('ref/return', 'FUNCTIONS'),
         'try': ('ref/try', 'EXCEPTIONS'),
         'while': ('ref/while', 'break continue if TRUTHVALUE'),
+        'yield': ('ref/yield', ''),
     }
 
     topics = {