Mention pg_index changes also cause relcache invalidation

Amit Langote, additional line by me

src/backend/utils/cache/inval.c

@@ -51,9 +51,9 @@
  *	PrepareToInvalidateCacheTuple() routine provides the knowledge of which
  *	catcaches may need invalidation for a given tuple.
  *
- *	Also, whenever we see an operation on a pg_class or pg_attribute tuple,
- *	we register a relcache flush operation for the relation described by that
- *	tuple.
+ *	Also, whenever we see an operation on a pg_class, pg_attribute, or
+ *	pg_index tuple, we register a relcache flush operation for the relation
+ *	described by that tuple (as specified in CacheInvalidateHeapTuple()).
  *
  *	We keep the relcache flush requests in lists separate from the catcache
  *	tuple flush requests.  This allows us to issue all the pending catcache
@@ -1132,6 +1132,7 @@ CacheInvalidateHeapTuple(Relation relation,
 
 	/*
 	 * Now, is this tuple one of the primary definers of a relcache entry?
+	 * See comments in file header for deeper explanation.
 	 *
 	 * Note we ignore newtuple here; we assume an update cannot move a tuple
 	 * from being part of one relcache entry to being part of another.