(help-argument-name): New face, inheriting from

(help-argument-name): New face, inheriting from
font-lock-variable-name-face, to highlight function arguments in
`describe-function' and `describe-key'.
(help-do-arg-highlight): Auxiliary function to highlight a given list of
arguments in a string.
(help-highlight-arguments): Highlight the function arguments and all uses
of them in the docstring.
(describe-function-1): Use it. Do docstring output via `insert', not
'princ', so text attributes are preserved.