doc: Add an overview of NULL treatment in PostgreSQL

The handling of NULL within PostgreSQL is broad, varied, and
unintuitive.  This new section provides a place for the reader
to go and skim over the various behaviors and stick them in
the back of their mind.

Some existing documentation is tweaked to be more in line with
the overall flow of the material presented in this overview.
The individual features do still constitute an authoritative
location for describing the behaviors - but now have a more
convenient place to put examples.

Author: polobo

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>