scripts: kernel-doc: reduce repeated regex expressions into variables

There are some regex expressions in the kernel-doc script, which are used
repeatedly in the script.

Reduce such expressions into variables, which can be used everywhere.

A quick manual check found that no errors and warnings were added/removed
in this process.

Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Aditya Srivastava <yashsri421@gmail.com>
Link: https://lore.kernel.org/r/20210514144244.25341-1-yashsri421@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>