Clarify foreign_table_where

The previous explanation for foreign_table_where was slightly
misleading. The examples speak for themselves, but they are
further down on the page.

The WHERE-clause is effectively appended to the existing WHERE
clause created by TYPO3 using the default values and must
start with AND.

Also, additional clauses such as ORDER BY or GROUP BY may be
added after the WHERE-clause.

Resolves: #663

Documentation/ColumnsConfig/Type/Select/Properties/ForeignTableWhere.rst

@@ -13,7 +13,11 @@ foreign_table_where
    :RenderType: all
 
    The items from :ref:`foreign_table <columns-select-properties-foreign-table>`
-   are selected with this WHERE-clause.
+   are selected with this WHERE-clause. The WHERE-clause is effectively
+   appended to the existing WHERE-clause (which contains default constraints,
+   such as 'NOT deleted') and must begin with AND. Additional clauses such as
+   ORDER BY may be added after the WHERE-clause. The result must be a valid SQL
+   statement.
 
 Field quoting
 =============