[DOCS] SELECT , UNION INTERSECT EXCEPT clause

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/sql-select.html
Description:

https://www.postgresql.org/docs/current/static/sql-select.html#SQL-UNION
In  this section there is information about usage ORDER BY and LIMIT inside
second UNION subquery. In my opinion, it's necessary to add information
about the same behavior of WITH clause and FOR and FETCH clauses.