bug#4402: apropos-documentation and platform-specific docs

[Glenn Morris <rgm@gnu.org>]

On GNU/Linux:
emacs -Q
M-x apropos-documentation nextstep

returns various matches, including things like:

x-create-frame
  Function: Make a new Nextstep window, called a "frame" in Emacs terms.
[...]

The one-line summary is incorrect, since it is the version specific to
the NS build, not the build currently running.

The problem is caused by the DOC file containing three sets of
doc-strings for Fx-create-frame, extracted from w32fns.c, xfns.c, and
nsfns.m.

This made me wonder, why does the DOC file in my GNU/Linux build
contain information extracted from Nextstep and Windows files that is
never going to be relevant? Perhaps it's so that one can in principle
share DOC files between different builds? This doesn't seem like a big
advantage to me. I doubt (m)any people have ever shared the DOC file
between any of these platforms. The size of DOC is a negligible part of
a current Emacs installation.

The standard solution to this kind of thing seems to be that where
functions/variables have multiple, platform-specific definitions, each
should have the same, unified doc-string. This is a bit of a pain.

Only putting the relevant files into the DOC file would eliminate this
issue.