KEYS: Add missing function documentation

Compiling with 'W=1' results in warnings that 'Function parameter or member
not described'

Add the missing parameters for
restrict_link_by_builtin_and_secondary_trusted and
restrict_link_to_builtin_trusted.

Use /* instead of /** for get_builtin_and_secondary_restriction, since
it is a static function.

Fix wrong function name restrict_link_to_builtin_trusted.

Fixes: d3bfe841

 ("certs: Add a secondary system keyring that can be added to dynamically")
Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>