Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()

There seems to be a significant amount of confusion around the new
try_cmpxchg(), despite this being more like the C11
atomic_compare_exchange_*() family. Add a few words of clarification
on how cmpxchg() and try_cmpxchg() relate to one another.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/YOMgPeMOmmiK3tXO@hirez.programming.kicks-ass.net