Comment about set_join_pathlist_hook()

Hi,

What I am concerned about from the report [1] is that this comment is
a bit too terse; it might cause a misunderstanding that extensions can
do different things than we intend to allow:

    /*
     * 6. Finally, give extensions a chance to manipulate the path list.
     */
    if (set_join_pathlist_hook)
        set_join_pathlist_hook(root, joinrel, outerrel, innerrel,
                               jointype, &extra);

So I would like to propose to extend the comment to explain what they
can do, as in the comment about set_rel_pathlist_hook() in allpaths.c.
Attached is a patch for that.

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/CACawEhV%3D%2BQ0HXrcDergbTR9EkVFukgRPMTZbRFL-YK5CRmvYag%40mail.gmail.com