Fix comment in heap.c

The description of InsertPgAttributeTuple() does not match its handling
of pg_attribute contents with NULL values for a long time, with 911e7020
making things more inconsistent.  This adjusts the description to match
the reality.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/4E4E4B33-9FDF-4D21-B77A-642D027AEAD9@yesql.se

src/backend/catalog/heap.c

@@ -714,8 +714,8 @@ CheckAttributeType(const char *attname,
  *		Construct and insert a new tuple in pg_attribute.
  *
  * Caller has already opened and locked pg_attribute.  new_attribute is the
- * attribute to insert.  attcacheoff is always initialized to -1, attacl and
- * attoptions are always initialized to NULL.
+ * attribute to insert.  attcacheoff is always initialized to -1, attacl,
+ * attfdwoptions and attmissingval are always initialized to NULL.
  *
  * indstate is the index state for CatalogTupleInsertWithInfo.  It can be
  * passed as NULL, in which case we'll fetch the necessary info.  (Don't do