doc: Document sequence function privileges better

Document the privileges required for each of the sequence functions.
This was already in the GRANT reference page, but also add it to the
function description for easier reference.

doc/src/sgml/func.sgml

@@ -11723,6 +11723,10 @@ nextval('foo'::text)      <lineannotation><literal>foo</literal> is looked up at
         </para>
        </important>
 
+       <para>
+        This function requires <literal>USAGE</literal>
+        or <literal>UPDATE</literal> privilege on the sequence.
+       </para>
       </listitem>
      </varlistentry>
 
@@ -11738,6 +11742,11 @@ nextval('foo'::text)      <lineannotation><literal>foo</literal> is looked up at
         other sessions have executed <function>nextval</function> since the
         current session did.
        </para>
+
+       <para>
+        This function requires <literal>USAGE</literal>
+        or <literal>SELECT</literal> privilege on the sequence.
+       </para>
       </listitem>
      </varlistentry>
 
@@ -11754,6 +11763,11 @@ nextval('foo'::text)      <lineannotation><literal>foo</literal> is looked up at
         <function>lastval</function> if <function>nextval</function>
         has not yet been called in the current session.
        </para>
+
+       <para>
+        This function requires <literal>USAGE</literal>
+        or <literal>SELECT</literal> privilege on the last used sequence.
+       </para>
       </listitem>
      </varlistentry>
 
@@ -11792,6 +11806,11 @@ SELECT setval('foo', 42, false);    <lineannotation>Next <function>nextval</> wi
          back.
         </para>
        </important>
+
+       <para>
+        This function requires <literal>UPDATE</literal> privilege on the
+        sequence.
+       </para>
       </listitem>
      </varlistentry>
     </variablelist>