bug#40900: Add face for function params in info

[Eli Zaretskii]

> Date: Mon, 27 Apr 2020 12:46:02 +0000
> From: ndame via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> The Elisp info manual could be easier to read if parameters which are 
> capitalized
> could have a face too:
> 
>    If the minibuffer prompt string PROMPT does not end in ‘:’ ...
> 
> 
> Though I don't know if it's possible to add language specific faces to info,
> because a capitalized word could mean something else in an other language.
> 
> Maybe a face like Info-elisp-param-face which is added only if the current
> info is about elisp?

I think the challenge is to reliably identify the strings that are
function parameters.  Info files lack the markup that identifies
various symbols in the Texinfo sources, and the code which finds
function arguments must not err too much, or users will complain.