mutex: improve header comment to be actually informative about the API

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: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>