Impact: improve documentation

It's nice to say that mutex_trylock follows the spin_trylock convention.
It's a lot nicer if the comment also says which that is...  make it so.
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>