doc: add a chapter about trylock functions [Bug 9011]

Add a chapter about trylock functions.
http://bugzilla.kernel.org/show_bug.cgi?id=9011



Signed-off-by: Matti Linnanvuori <mattilinnanvuori@yahoo.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (removed down_trylock)