Return better output on webapi doc parse_text failures.
When docs are missing from a class or function, parse_text will fail

accessing a function on a None. That's not useful, and the error output

doesn't tell us what happened or where.
Now, most callers provide a 'where' string that's used for any assertion

messages to better let us know what was missing.

Encountered this problem in another change. This change let me know where

the failure was.