Dial back -Wimplicit-fallthrough to level 3

The additional pain from level 4 is excessive for the gain.

Also revert all the source annotation changes to their original
wordings, to avoid back-patching pain.

Discussion: https://postgr.es/m/31166.1589378554@sss.pgh.pa.us

src/backend/replication/walreceiver.c

@@ -215,7 +215,7 @@ WalReceiverMain(void)
 		case WALRCV_STOPPING:
 			/* If we've already been requested to stop, don't start up. */
 			walrcv->walRcvState = WALRCV_STOPPED;
-			/* FALLTHROUGH */
+			/* fall through */
 
 		case WALRCV_STOPPED:
 			SpinLockRelease(&walrcv->mutex);