Fix inadequacies in recently added wait events

In commit 9915de6c1c, we introduced a new wait point for replication
slots and incorrectly labelled it as wait event PG_WAIT_LOCK.  That's
wrong, so invent an appropriate new wait event instead, and document it
properly.

While at it, fix numerous other problems in the vicinity:
- two different walreceiver wait events were being mixed up in a single
  wait event (which wasn't documented either); split it out so that they
  can be distinguished, and document the new events properly.

- ParallelBitmapPopulate was documented but didn't exist.

- ParallelBitmapScan was not documented (I think this should be called
  "ParallelBitmapScanInit" instead.)

- Logical replication wait events weren't documented

- various symbols had been added in dartboard order in various places.
  Put them in alphabetical order instead, as was originally intended.

Discussion: https://postgr.es/m/20170808181131.mu4fjepuh5m75cyq@alvherre.pgsql

src/backend/replication/slot.c

@@ -391,7 +391,8 @@ retry:
 							name, active_pid)));
 
 		/* Wait here until we get signaled, and then restart */
-		ConditionVariableSleep(&slot->active_cv, PG_WAIT_LOCK);
+		ConditionVariableSleep(&slot->active_cv,
+							   WAIT_EVENT_REPLICATION_SLOT_DROP);
 		ConditionVariableCancelSleep();
 		goto retry;
 	}