docs: kdoc: handle the obsolescensce of docutils.ErrorString()

commit 00d95fcc upstream.

The ErrorString() and SafeString() docutils functions were helpers meant to
ease the handling of encodings during the Python 3 transition.  There is no
real need for them after Python 3.6, and docutils 0.22 removes them,
breaking the docs build

Handle this by just injecting our own one-liner version of ErrorString(),
and removing the sole SafeString() call entirely.

Reported-by: Zhixu Liu <zhixu.liu@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <87ldmnv2pi.fsf@trenco.lwn.net>
[ Salvatore Bonaccorso: Backport to v6.17.y for context changes in
  Documentation/sphinx/kernel_include.py with major refactorings for the v6.18
  development cycle. Backport ErrorString definition as well to
  Documentation/sphinx/kernel_abi.py file for 6.12.y where it is imported
  from docutils before the faccc0ec ("docs: sphinx/kernel_abi: adjust
  coding style") change. ]
Suggested-by: Andreas Radke <andreas.radke@mailbox.org>
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>