Update comments about progress reporting by index_drop

Michaël Paquier complained that index_drop is requesting progress
reporting for non-obvious reasons, so let's add a comment to explain
why.

Discussion: https://postgr.es/m/20191017010412.GH2602@paquier.xyz

src/backend/catalog/index.c

@@ -2142,6 +2142,10 @@ index_drop(Oid indexId, bool concurrent, bool concurrent_lock_mode)
 		 * possible if one of the transactions in question is blocked trying
 		 * to acquire an exclusive lock on our table.  The lock code will
 		 * detect deadlock and error out properly.
+		 *
+		 * Note: we report progress through WaitForLockers() unconditionally
+		 * here, even though it will only be used when we're called by REINDEX
+		 * CONCURRENTLY and not when called by DROP INDEX CONCURRENTLY.
 		 */
 		WaitForLockers(heaplocktag, AccessExclusiveLock, true);
 
@@ -2157,7 +2161,7 @@ index_drop(Oid indexId, bool concurrent, bool concurrent_lock_mode)
 
 		/*
 		 * Wait till every transaction that saw the old index state has
-		 * finished.
+		 * finished.  See above about progress reporting.
 		 */
 		WaitForLockers(heaplocktag, AccessExclusiveLock, true);