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

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

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>
Upstream: https://git.kernel.org/linus/00d95fcc4dee66dfb6980de6f2973b32f973a1eb
[j.ne: Adapted from Linux to U-Boot]
Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>