In ALTER EXTENSION docs, document left_type/right_type.

Noted by Daniele Varrazzo.

doc/src/sgml/ref/alter_extension.sgml

@@ -241,6 +241,18 @@ ALTER EXTENSION <replaceable class="PARAMETER">extension_name</replaceable> DROP
      </listitem>
     </varlistentry>
 
+    <varlistentry>
+     <term><replaceable class="parameter">left_type</replaceable></term>
+     <term><replaceable class="parameter">right_type</replaceable></term>
+     <listitem>
+      <para>
+       The data type(s) of the operator's arguments (optionally
+       schema-qualified).  Write <literal>NONE</> for the missing argument
+       of a prefix or postfix operator.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry>
      <term><literal>PROCEDURAL</literal></term>