Correct SELECT syntax drilldown

The syntax drill-down for the SELECT statement is incorrect:

Select terms: if there are no set operators, the only permitted select term is a subselect. If there is a set operator, both select terms before and after may be a subselect or a subquery expression. (Rename subselect to "query block".)

WITH clause: for all versions from 6.5 to 7.0, the WITH clause should go at the start of the query block: FROM-SELECT or SELECT-FROM. In 7.1, the WITH clause is now at the start of the SELECT statement, before the select terms. (There was no WITH clause in 6.0.) Move WITH in the navigation accordingly.

Pagination: in all versions, the OFFSET clause may alternatively appear before the LIMIT clause.

For 7.1 perhaps add a note saying that the EBNF is not normative.