add descriptions 6-2

Author: v-thpra

docs/relational-databases/xml/columns-that-contain-a-null-value-by-default.md

@@ -1,5 +1,6 @@
 ---
 title: "Columns that Contain a Null Value By Default | Microsoft Docs"
+description: Learn how to work with columns that contain a null value by default by using the ELEMENTS XSINIL keyword phrase in SQL.
 ms.custom: "fresh2019may"
 ms.date: "05/22/2019"
 ms.prod: sql

docs/relational-databases/xml/generate-siblings-with-a-nested-auto-mode-query.md

@@ -1,5 +1,6 @@
 ---
 title: "Generate Siblings with a Nested AUTO Mode Query | Microsoft Docs"
+description: View an example of how to generate siblings by using a nested AUTO mode query as an alternative to using EXPLICIT mode to generate the same XML.
 ms.custom: ""
 ms.date: "03/04/2017"
 ms.prod: sql

docs/xquery/constructor-functions-xquery.md

@@ -1,5 +1,6 @@
 ---
 title: "Constructor Functions (XQuery) | Microsoft Docs"
+description: Learn about the constructor functions in XQuery, that allow you to create instances of any of the XSD built-in or user-defined atomic types.
 ms.custom: ""
 ms.date: "03/09/2017"
 ms.prod: sql

docs/xquery/path-expressions-specifying-predicates.md

@@ -1,5 +1,6 @@
 ---
 title: "Specifying Predicates in a Path Expression Step | Microsoft Docs"
+description: Learn about specifying predicates in the axis step of an XQuery PATH expression to filter an XML node sequence.
 ms.custom: ""
 ms.date: "03/17/2017"
 ms.prod: sql