docs: parse-headers.py: better handle @var arguments

docs: parse-headers.py: better handle @var arguments

The kernel-doc markups inside headers may contain @var markups.

With the current rule, this would be converted into:

     \* @:c:type:`DMX_BUFFER_FLAG_DISCONTINUITY_INDICATOR <dmx_buffer_flags>`\:

Fix it adding a non-printed space if needed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/8d06bb713d6ec8de65179dd93defe479715409b6.1755872208.git.mchehab+huawei@kernel.org