Add section IDs to subsections of syntax.sgml that lacked them.

This is so that these sections will have stable HTML tags that one can
link to, rather than things like "AEN1902".  Perhaps we should mount a
campaign to do this everywhere, but I've found myself pointing at
syntax.sgml subsections often enough to be sure it's useful here.

doc/src/sgml/syntax.sgml

@@ -664,7 +664,7 @@ $function$
     </para>
    </sect3>
 
-   <sect3>
+   <sect3 id="sql-syntax-constants-numeric">
     <title>Numeric Constants</title>
 
     <indexterm>
@@ -852,7 +852,7 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
    </para>
   </sect2>
 
-  <sect2>
+  <sect2 id="sql-syntax-special-chars">
    <title>Special Characters</title>
 
   <para>
@@ -1310,7 +1310,7 @@ SELECT 3 OPERATOR(pg_catalog.+) 4;
    the remaining options.
   </para>
 
-  <sect2>
+  <sect2 id="sql-expressions-column-refs">
    <title>Column References</title>
 
    <indexterm>
@@ -1333,7 +1333,7 @@ SELECT 3 OPERATOR(pg_catalog.+) 4;
    </para>
   </sect2>
 
-  <sect2>
+  <sect2 id="sql-expressions-parameters-positional">
    <title>Positional Parameters</title>
 
    <indexterm>
@@ -1373,7 +1373,7 @@ CREATE FUNCTION dept(text) RETURNS dept
    </para>
   </sect2>
 
-  <sect2>
+  <sect2 id="sql-expressions-subscripts">
    <title>Subscripts</title>
 
    <indexterm>
@@ -1468,7 +1468,7 @@ $1.somecolumn
    </para>
   </sect2>
 
-  <sect2>
+  <sect2 id="sql-expressions-operator-calls">
    <title>Operator Invocations</title>
 
    <indexterm>
@@ -1497,7 +1497,7 @@ $1.somecolumn
    </para>
   </sect2>
 
-  <sect2>
+  <sect2 id="sql-expressions-function-calls">
    <title>Function Calls</title>
 
    <indexterm>