bug#31441: 26; doc of functions `(lax-)plist-get'

[Drew Adams <drew.adams@oracle.com>]

The doc in (elisp) `Plist Access' is mostly OK, but it could
be improved.  The doc strings of these two functions are not OK.

The doc strings need to point out, as does the manual, that `plist-get'
uses `eq' and `lax-plist-get' uses `equal'.  In addition, the doc string
of `lax-plist-get' should, like the manual, say that it is in all other
respects just like `plist-get' (providing a link to that function's
description).

In addition the manual description of `plist-get' could be improved by
mentioning these two things, which are mentioned by its doc string:

1. It uses `eq'.  (If one reads further then this becomes apparent when
   reading about `lax-plist-get'.

2. "This function never signals an error.  This function does not change
   global state, including the match data."  If that is worth saying in
   the doc string (dunno) then it is worth saying in the manual,
   especially as the description of `lax-plist-get' says that it is like
   `plist-get' except for using `equal'.



In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-04-10
Repository revision: c267421647510319d2a70554e42f0d1c394dba0a
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''