Update GROUP BY ALL comments about window functions

Update GROUP BY ALL comments about window functions

When GROUP BY ALL was added in commit ef38a4d9756, the SQL standard
working draft was silent on what to do with window functions.  This
has now been fixed in the SQL standard working draft.  Update the
documentation and code comments about that.

Also make the documentation more specific that we are only talking
about aggregate functions referring to the same query level, which is
another thing that has been made more precise in the SQL standard
working draft since.

The PostgreSQL implementation was already doing the right thing for
both aspects, so no functionality changes.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/CAHM0NXjz0kDwtzoe-fnHAqPB1qA8_VJN0XAmCgUZ%2BiPnvP5LbA%40mail.gmail.com