Documentation/atomic_t: Clarify signed vs unsigned

Clarify the whole signed vs unsigned issue for atomic_t.

There has been enough confusion on this topic to warrant a few explicit
words I feel.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>