Remove cautions about using volatile from spin.h.

Commit 0709b7ee obsoleted this comment
but neglected to update it.

Thomas Munro

src/include/storage/spin.h

@@ -25,15 +25,10 @@
  *	Callers must beware that the macro argument may be evaluated multiple
  *	times!
  *
- *	CAUTION: Care must be taken to ensure that loads and stores of
- *	shared memory values are not rearranged around spinlock acquire
- *	and release. This is done using the "volatile" qualifier: the C
- *	standard states that loads and stores of volatile objects cannot
- *	be rearranged *with respect to other volatile objects*. The
- *	spinlock is always written through a volatile pointer by the
- *	spinlock macros, but this is not sufficient by itself: code that
- *	protects shared data with a spinlock MUST reference that shared
- *	data through a volatile pointer.
+ *	Load and store operations in calling code are guaranteed not to be
+ *	reordered with respect to these operations, because they include a
+ *	compiler barrier.  (Before PostgreSQL 9.5, callers needed to use a
+ *	volatile qualifier to access data protected by spinlocks.)
  *
  *	Keep in mind the coding rule that spinlocks must not be held for more
  *	than a few instructions.  In particular, we assume it is not possible