Document JSON primitive quoting and letter-case rules

[David G. Johnston]

In Bug# 11636 [1] ChandraKumar complained that we documented fairly
explicitly how to write json boolean true and false but not json null.
 While repeating the entire RFC in our docs isn't desirable its seems worth
considering making the comments on the existing table consistently cover
quoting and letter-case rules for the four primitive types shown.  Thus the
attached.

David J.

[1]
https://www.postgresql.org/message-id/CAHyU1EYRYCV8gELrBYm6V8E%2BToqf%3DKQe370bPt4yrE_Y1yDZ9w%40mail.gmail.com

commit e009aab5ad4cb7852fec91d759aef3f356542a5e
Author: David G. Johnston <david.g.johnston@gmail.com>
Date:   Wed Sep 30 02:46:26 2020 +0000

    Don't make reader view RFC for scalar json value syntax

diff --git a/doc/src/sgml/json.sgml b/doc/src/sgml/json.sgml
index c0a6554d4d..d727ffc669 100644
--- a/doc/src/sgml/json.sgml
+++ b/doc/src/sgml/json.sgml
@@ -147,22 +147,22 @@
         <entry><type>string</type></entry>
         <entry><type>text</type></entry>
         <entry><literal>\u0000</literal> is disallowed, as are Unicode escapes
-         representing characters not available in the database encoding</entry>
+         representing characters not available in the database encoding.  JSON values are double-quoted.</entry>
        </row>
        <row>
         <entry><type>number</type></entry>
         <entry><type>numeric</type></entry>
-        <entry><literal>NaN</literal> and <literal>infinity</literal> values are disallowed</entry>
+        <entry><literal>NaN</literal> and <literal>infinity</literal> values are disallowed.  JSON values are unquoted.</entry>
        </row>
        <row>
         <entry><type>boolean</type></entry>
         <entry><type>boolean</type></entry>
-        <entry>Only lowercase <literal>true</literal> and <literal>false</literal> spellings are accepted</entry>
+        <entry>Only unquoted lowercase <literal>true</literal> and <literal>false</literal> spellings are accepted</entry>
        </row>
        <row>
         <entry><type>null</type></entry>
         <entry>(none)</entry>
-        <entry>SQL <literal>NULL</literal> is a different concept</entry>
+        <entry>SQL <literal>NULL</literal> is a different concept.  Must be written lowercase and unquoted.</entry>
        </row>
       </tbody>
      </tgroup>