__implemented_asyncs.
(lexer): Ignore @defcodeindex and @syncodeindex.
(lexer.toplevel_node): Check for undocumentet arguments to
	asynchronous messages.
(lexer.toplevel_findex): Check for @amindex entries in @findex nodes.
(lexer.toplevel_amindex): New method.
(lexer.__parse_async): New method.
(lexer.toplevel_aarg): Implemented.
(lexer.toplevel_bye): Check for unterminated @amindex nodes and
	undocumented asynchronous messages.