docs: parse-headers.py: simplify the rules for hashes

docs: parse-headers.py: simplify the rules for hashes

Normal :ref domain accept either hashes or underscores, but
c-domain ones don't. Fix it and remove unneeded places where
we opt to disable underscore transformation.

Ideally, we should have a rule about the default, or change
the way media docs have their references.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/0c011090272f7a1068545409222f970ddb1ed431.1755872208.git.mchehab+huawei@kernel.org