doc-guide: kernel-doc: specify that W=n does not check header files

doc-guide: kernel-doc: specify that W=n does not check header files

The documentation states that:

  make W=n

can be used to verify the documentation format. This is true for .c
files but not for headers [1].

Modify the documentation to specify that headers files are not covered
by make W=n and that these need to be checked separately with
scripts/kernel-doc.

[1] commit 3a025e1d1c2e ("Add optional check for bad kernel-doc comments")
Link: https://git.kernel.org/torvalds/c/3a025e1d1c2e
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251225-doc-format-check-v1-1-dff637a4d275@kernel.org>