Adjust conflicting documentation about \s vis-à-vis \cK

# New Ticket Created by  Thomas Sibley
# Please include the string:  [perl #121558]
# in the subject line of all future correspondence about this issue.
# <URL: https://rt.perl.org/Ticket/Display.html?id=121558 >

See attached patch against blead.

>From 84a704858995060ef78a39372ca7e2572c6141a3 Mon Sep 17 00:00:00 2001
From: Thomas Sibley <tsibley@cpan.org>
Date: Tue, 1 Apr 2014 13:49:26 -0700
Subject: [PATCH] =?UTF-8?q?Adjust=20conflicting=20documentation=20about=20?=
 =?UTF-8?q?\s=20vis-=C3=A0-vis=20\cK?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

It makes little sense to say that \s matches \cK unconditionally in the
same sentence as saying it matches \cK conditionally (>= Perl 5.18).

I suppose the intention could have been to specify the full char class
in the documentation from 5.18 onwards while still noting when that
change happened (the introducing commit, d28d802, was only on 5.17+),
but the sentence structure and convention in the rest of the document
suggests otherwise.
Fix comments and pod that mention 5.20 erroneously

In certain places in the documentation, "5.20" is no longer applicable.
Also, a message referred to in perldiag got reworded, but our checks did
not catch that perldiag should have been updated.