bug#35498: 26.2; doc string of `isearch-regexp-function'

[Eli Zaretskii]

> Date: Mon, 29 Apr 2019 18:48:48 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> The doc string should describe the possible values and their
> meanings/behaviors.  Currently it describes only a function as value.

I added the existing functions that could be used as values.  Their
descriptions are available by using the hyperlink, so no need to
duplicate that here.  The list is, of course, not exhaustive, as Lisp
programs could define their own functions.

> Searching isearch.el it seems that this variable can be any of the
> values possible for `search-default-mode'.  Please describe each of the
> values for `isearch-regexp-function'.

See above: not needed.

> Even when the value is a function the description could be better,
> saying that it must accept a string as its first arg and must return a
> string.  That can be surmised from saying that it converts a plain
> search string to a regexp [search string], but it could be clearer.

Added.

> Also, "plain search string" and "regexp" are unclear here.  The
> distinction is presumably (?) that the former contains no regexp special
> chars?  Or does the return value need to be a _valid_ (complete) regexp,
> and does the function raise an error if that's not possible?

I didn't see any problem here, although I improved the wording to some
extent.

Thanks.