Adjust comment about timeout when waiting for WAL at recovery

A timeout of 5s is used when waiting for WAL to become available at
recovery so as the startup process is able to react promptly if a
trigger file shows up.  However this missed the fact that the startup
process also relies on the timeout to check periodically the status of
any active WAL receiver.

Discussion: https://postgr.es/m/20190131070956.GE13429@paquier.xyz

src/backend/access/transam/xlog.c

@@ -12104,7 +12104,8 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
 
 					/*
 					 * Wait for more WAL to arrive. Time out after 5 seconds
-					 * to react to a trigger file promptly.
+					 * to react to a trigger file promptly and to check if the
+					 * WAL receiver is still active.
 					 */
 					(void) WaitLatch(&XLogCtl->recoveryWakeupLatch,
 									 WL_LATCH_SET | WL_TIMEOUT |