locking/atomic: atomic: remove stale comments

The commentary in asm-generic/atomic.h is stale; let's bring it up-to
date:

* The block comment at the start of the file mentions this is only
  usable on UP systems, but is immediately followed by an SMP
  implementation using cmpxchg. Let's delete the misleading statement.

* A comment near the end of the file was originally at the top of the
  file, but over time rework has shuffled it near the end, and it's long
  been superceded by the block comment at the top of the file. Let's
  remove it.

* Since asm-generic/atomic.h isn't the canonical documentation for the
  atomic ops, and since the existing comments are not in kerneldoc
  format, we don't need to document the semantics of each operation here
  (and this would be better done in a centralised document). Let's
  remove these comments.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210525140232.53872-7-mark.rutland@arm.com