docs: automarkup.py: Fix regexes to solve sphinx 3 warnings

With the transition to Sphinx 3, new warnings were generated by
automarkup, exposing bugs in the regexes.

The warnings were caused by the expressions matching words in the
translated versions of the documentation, since any unicode character
was matched.

Fix the regular expression by making the C regexes use ASCII and
ensuring the expressions only match the beginning of words,
in order to avoid warnings like this:

	WARNING: Unparseable C cross-reference: '调用debugfs_rename'

That's probably due to the lack of using spaces between words
on Chinese.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>