bug#30068: 26.0; REGRESSION: no doc for `x-display-pixel-*'

[Eli Zaretskii]

> Date: Thu, 11 Jan 2018 22:19:18 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 30068@debbugs.gnu.org
> 
> > From: Glenn Morris <rgm@gnu.org>
> > Cc: 30068@debbugs.gnu.org
> > Date: Thu, 11 Jan 2018 14:26:02 -0500
> > 
> > Keep the real doc only in xfns.c, makes the others just say <skip>, or
> > <see:xfns.c>, and change Fsnarf_documentation to respect <skip>.
> 
> Sounds fine.  Are there any downsides?

Btw, there's another, perhaps easier alternative: have only one
definition of the function, with the single copy of the doc string,
and then make it call the platform-dependent parts on the C level.