Add comment to explain why PGReserveSemaphores() is called early

Before commit e25626677f, PGReserveSemaphores() had to be called
before SpinlockSemaInit() because spinlocks were implemented using
semaphores on some platforms (--disable-spinlocks). Add a comment
explaining that.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAExHW5seSZpPx-znjidVZNzdagGHOk06F+Ds88MpPUbxd1kTaA@mail.gmail.com
Backpatch-to: 18

src/backend/storage/ipc/ipci.c

@@ -227,7 +227,9 @@ CreateSharedMemoryAndSemaphores(void)
 	InitShmemAccess(seghdr);
 
 	/*
-	 * Create semaphores
+	 * Create semaphores.  (This is done here for historical reasons.  We used
+	 * to support emulating spinlocks with semaphores, which required
+	 * initializing semaphores early.)
 	 */
 	PGReserveSemaphores(numSemas);