Documentation/features/locking: update lists

The locking feature lists don't match reality as of v5.3-rc1:

* arm64 moved to queued spinlocks in commit:
  c1109047
  ("arm64: locking: Replace ticket lock implementation with qspinlock")

* xtensa moved to queued spinlocks and rwlocks in commit:
  579afe86
  ("xtensa: use generic spinlock/rwlock implementation")

* architecture-specific rwsem support was removed in commit:
  46ad0840


  ("locking/rwsem: Remove arch specific rwsem files")

So update the feature lists accordingly, and remove the now redundant
rwsem-optimized list.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>