Improve comment for OAT_POST_CREATE.

Clarify that the command counter may or may not have been incremented.

We may want to change the behavior to be more consistent, but until
that time, at least improve the comment.

Discussion: https://postgr.es/m/CAHoZxqvN2eoic_CvjsAvpryyLyA2xG8JmsyMtKFFJz_1oFhfOg%40mail.gmail.com
Reported-by: Mary Xu

src/include/catalog/objectaccess.h

@@ -17,7 +17,9 @@
  *
  * OAT_POST_CREATE should be invoked just after the object is created.
  * Typically, this is done after inserting the primary catalog records and
- * associated dependencies.
+ * associated dependencies. The command counter may or may not be incremented
+ * at the time the hook is invoked; if not, the extension can use SnapshotSelf
+ * to get the new version of the tuple.
  *
  * OAT_DROP should be invoked just before deletion of objects; typically
  * deleteOneObject(). Its arguments are packed within ObjectAccessDrop.