Doc: list AT TIME ZONE and COLLATE in operator precedence table.

These constructs have precedence, but we forgot to list them.
In HEAD, mention AT LOCAL as well as AT TIME ZONE.

Per gripe from Shay Rojansky.

Discussion: https://postgr.es/m/CADT4RqBPdbsZW7HS1jJP319TMRHs1hzUiP=iRJYR6UqgHCrgNQ@mail.gmail.com

doc/src/sgml/syntax.sgml

@@ -1021,6 +1021,18 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
        <entry>unary plus, unary minus</entry>
       </row>
 
+      <row>
+       <entry><token>COLLATE</token></entry>
+       <entry>left</entry>
+       <entry>collation selection</entry>
+      </row>
+
+      <row>
+       <entry><token>AT</token></entry>
+       <entry>left</entry>
+       <entry><literal>AT TIME ZONE</literal></entry>
+      </row>
+
       <row>
        <entry><token>^</token></entry>
        <entry>left</entry>