Fix typos in comments

The changes done in this commit impact comments with no direct
user-visible changes, with fixes for incorrect function, variable or
structure names.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/e8c38840-596a-83d6-bd8d-cebc51111572@gmail.com

src/backend/storage/file/fd.c

@@ -3069,7 +3069,7 @@ AtEOXact_Files(bool isCommit)
 /*
  * BeforeShmemExit_Files
  *
- * before_shmem_access hook to clean up temp files during backend shutdown.
+ * before_shmem_exit hook to clean up temp files during backend shutdown.
  * Here, we want to clean up *all* temp files including interXact ones.
  */
 static void

src/backend/storage/ipc/latch.c

@@ -151,7 +151,7 @@ struct WaitEventSet
 #endif
 };
 
-/* A common WaitEventSet used to implement WatchLatch() */
+/* A common WaitEventSet used to implement WaitLatch() */
 static WaitEventSet *LatchWaitSet;
 
 /* The position of the latch in LatchWaitSet. */

src/backend/storage/ipc/procarray.c

@@ -1754,7 +1754,7 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
 		 * current top-level xid any.
 		 *
 		 * Without an assigned xid we could use a horizon as aggressive as
-		 * ReadNewTransactionid(), but we can get away with the much cheaper
+		 * GetNewTransactionId(), but we can get away with the much cheaper
 		 * latestCompletedXid + 1: If this backend has no xid there, by
 		 * definition, can't be any newer changes in the temp table than
 		 * latestCompletedXid.

src/backend/storage/lmgr/spin.c

@@ -37,7 +37,7 @@
 #define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES)
 #else
 #define NUM_EMULATION_SEMAPHORES (NUM_SPINLOCK_SEMAPHORES)
-#endif							/* DISABLE_ATOMICS */
+#endif							/* HAVE_ATOMICS */
 
 PGSemaphore *SpinlockSemaArray;