Fix comments in heaptuple.c

Since e27f4ee, fastgetattr() and heap_getattr() are not macros, but
inlined functions.

Author: Junwang Zhao
Reviewed-by: Stepan Neretin
Discussion: https://postgr.es/m/CAEG8a3JS-JKWWyOcM7BU=vPqFXa3W7mZSHnvc3CBqx=tC+3SCA@mail.gmail.com

Author: michaelpq

src/backend/access/common/heaptuple.c

@@ -495,8 +495,8 @@ heap_attisnull(HeapTuple tup, int attnum, TupleDesc tupleDesc)
 /* ----------------
  *		nocachegetattr
  *
- *		This only gets called from fastgetattr() macro, in cases where
- *		we can't use a cacheoffset and the value is not null.
+ *		This only gets called from fastgetattr(), in cases where we
+ *		can't use a cacheoffset and the value is not null.
  *
  *		This caches attribute offsets in the attribute descriptor.
  *
@@ -715,8 +715,8 @@ nocachegetattr(HeapTuple tup,
  *
  *		Fetch the value of a system attribute for a tuple.
  *
- * This is a support routine for the heap_getattr macro.  The macro
- * has already determined that the attnum refers to a system attribute.
+ * This is a support routine for heap_getattr().  The function has already
+ * determined that the attnum refers to a system attribute.
  * ----------------
  */
 Datum