Fix argument name highlighting in 'describe-function' output

Fix argument name highlighting in 'describe-function' output

This fixes a regression introduced in
dd38097f0b9458eea882043fa06b27248e505b22, which prevented proper
argument name highlighting in function docstrings in the output
buffer of 'describe-function'.

* lisp/help-fns.el (help-do-arg-highlight): New optional arg
that says if we're highlighting arguments in the usage string or
in the docstring.  Tweak generated regex to only expect argument
name to be preceded by function name in usage string.
(help-highlight-arguments): Use new argument for usage string.