Include unary plus in the Operator Precedence table.

Per gripe from Grzegorz Szpetkowski.

Also, change the subsection heading from "Lexical Precedence" (which is
a contradiction in terms) to "Operator Precedence".

doc/src/sgml/syntax.sgml

@@ -972,7 +972,7 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
   </sect2>
 
   <sect2 id="sql-precedence">
-   <title>Lexical Precedence</title>
+   <title>Operator Precedence</title>
 
    <indexterm zone="sql-precedence">
     <primary>operator</primary>
@@ -1039,9 +1039,9 @@ SELECT (5 !) - 6;
       </row>
 
       <row>
-       <entry><token>-</token></entry>
+       <entry><token>+</token> <token>-</token></entry>
        <entry>right</entry>
-       <entry>unary minus</entry>
+       <entry>unary plus, unary minus</entry>
       </row>
 
       <row>
@@ -1065,7 +1065,7 @@ SELECT (5 !) - 6;
       <row>
        <entry><token>IS</token></entry>
        <entry></entry>
-       <entry><literal>IS TRUE</>, <literal>IS FALSE</>, <literal>IS UNKNOWN</>, <literal>IS NULL</></entry>
+       <entry><literal>IS TRUE</>, <literal>IS FALSE</>, <literal>IS NULL</>, etc</entry>
       </row>
 
       <row>