[CF 5086] v7 - Document NULL Concepts and Behaviors

This branch was automatically generated by a robot using patches from an
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https://commitfest.postgresql.org/patch/5086

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Patch(es): https://www.postgresql.org/message-id/CAKFQuwawRTo6pVmyj2FQqKKjgiV=XPU5PTu07yNJoE_gGt_OBg@mail.gmail.com
Author(s): David Johnston

Author: Commitfest Bot

doc/src/sgml/datatype.sgml

@@ -5389,7 +5389,7 @@ WHERE ...
        <row>
         <entry><type>unknown</type></entry>
         <entry>Identifies a not-yet-resolved type, e.g., of an undecorated
-         string literal.</entry>
+         string literal.  Also, the <link linkend="nullvalues-usage">null value.</link></entry>
        </row>
       </tbody>
      </tgroup>

doc/src/sgml/ddl.sgml

@@ -168,6 +168,8 @@ DROP TABLE products;
   </para>
  </sect1>
 
+ &nullvalues;
+
  <sect1 id="ddl-default">
   <title>Default Values</title>
 

doc/src/sgml/filelist.sgml

@@ -21,6 +21,7 @@
 <!ENTITY indices    SYSTEM "indices.sgml">
 <!ENTITY json       SYSTEM "json.sgml">
 <!ENTITY mvcc       SYSTEM "mvcc.sgml">
+<!ENTITY nullvalues SYSTEM "nullvalues.sgml">
 <!ENTITY parallel   SYSTEM "parallel.sgml">
 <!ENTITY perform    SYSTEM "perform.sgml">
 <!ENTITY queries    SYSTEM "queries.sgml">

doc/src/sgml/func.sgml

@@ -129,6 +129,11 @@
   the corresponding <productname>PostgreSQL</productname> types.
  </para>
 
+ <indexterm>
+  <primary>null value</primary>
+  <secondary sortas="json">within JSON</secondary>
+ </indexterm>
+
   <table id="json-type-mapping-table">
      <title>JSON Primitive Types and Corresponding <productname>PostgreSQL</productname> Types</title>
      <tgroup cols="3">
@@ -162,7 +167,7 @@
        <row>
         <entry><type>null</type></entry>
         <entry>(none)</entry>
-        <entry>SQL <literal>NULL</literal> is a different concept</entry>
+        <entry>An SQL null value is similar, but see <xref linkend="nullvalues-json"/> for differences.</entry>
        </row>
       </tbody>
      </tgroup>