kernel-doc: Add unary operator * to $type_param_ref

In kernel-doc comments, unary operator * collides with Sphinx/
docutil's markdown for emphasizing.

This resulted in additional warnings from "make htmldocs":

    WARNING: Inline emphasis start-string without end-string.

, as reported recently [1].

Those have been worked around either by escaping * (like \*param) or by
using inline-literal form of ``*param``, both of which are specific
to Sphinx/docutils.

Such workarounds are against the kenrel-doc's ideal and should better
be avoided.

Instead, add "*" to the list of unary operators kernel-doc recognizes
and make the form of *@param available in kernel-doc comments.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: [1] https://lore.kernel.org/r/20240223153636.41358be5@canb.auug.org.au/


Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>