Fix comments in StrategyNotifyBgWriter().

The interface for the function was changed in
d72731a7 but the comments of the
function was not updated.

Patch by Yugo Nagata.

src/backend/storage/buffer/freelist.c

@@ -406,8 +406,8 @@ StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc)
 /*
  * StrategyNotifyBgWriter -- set or clear allocation notification latch
  *
- * If bgwriterLatch isn't NULL, the next invocation of StrategyGetBuffer will
- * set that latch.  Pass NULL to clear the pending notification before it
+ * If bgwprocno isn't -1, the next invocation of StrategyGetBuffer will
+ * set that latch.  Pass -1 to clear the pending notification before it
  * happens.  This feature is used by the bgwriter process to wake itself up
  * from hibernation, and is not meant for anybody else to use.
  */