scripts: kernel-doc: accept negation like !@var

scripts: kernel-doc: accept negation like !@var

On a few places, it sometimes need to indicate a negation of a
parameter, like:

	!@fshared

This pattern happens, for example, at:

	kernel/futex.c

and it is perfectly valid. However, kernel-doc currently
transforms it into:

	!**fshared**

This won't do what it would be expected.

Fortunately, fixing the script is a simple matter of storing
the "!" before "@" and adding it after the bold markup, like:

	**!fshared**

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/0314b47f8c3e1f9db00d5375a73dc3cddd8a21f2.1586881715.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>