bug#24694: Document url--allow-chars for external use?

[Robert Pluim]

>>>>> On Fri, 11 Oct 2019 15:38:03 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> From: Hong Xu <hong@topbug.net>
    >> Cc: 24694@debbugs.gnu.org
    >> Date: Fri, 4 Oct 2019 12:31:27 -0700
    >> 
    >> > Make url-hexify-string accept a list of allowed chars.
    >> > 
    >> >        * url-util.el (url-hexify-string): Accept a list of allowed 
chars.
    >> >        * url.texi (URI Encoding): Update url-hexify-string doc and 
index improvements.
    >> > 
    >> 
    >> It's been a few years... I'm wondering whether this patch is still 
interesting?

    Eli> I think it is, yes.  Can you send a patch relative to the current
    Eli> master branch?

Api nit: if the first argument is going to be a list or a vector, you
could just keep one argument and check 'vectorp'.

Robert