Update obsolete comments in lwlock.h.

The typical size of an LWLock is now 16 bytes even on 64-bit platforms,
and the size of slock_t is now irrelevant.  But pg_atomic_uint32 can
(perhaps surprisingly) still be larger than 4 bytes, so there's still
some marginal point to allowing LWLOCK_MINIMAL_SIZE == 64.

Commit 008608b9 made the changes
that led to the need for these updates.