fs: anon_inodes: rephrase to appropriate kernel-doc

Commit e7e832ce ("fs: add LSM-supporting anon-inode interface") adds
more kerneldoc description, but also a few new warnings on
anon_inode_getfd_secure() due to missing parameter descriptions.

Rephrase to appropriate kernel-doc for anon_inode_getfd_secure().

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>