docs: kdoc: remove the KernelEntry::is_kernel_comment member

docs: kdoc: remove the KernelEntry::is_kernel_comment member

entry::is_kernel_comment never had anything to do with the entry itself; it
is a bit of local state in one branch of process_name().  It can, in fact,
be removed entirely; rework the code slightly so that it is no longer
needed.

No change in the rendered output.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250606163438.229916-6-corbet@lwn.net