Add documentation for opclass options

911e702077 added opclass options and adjusted documentation for each
particular affected opclass.  However, documentation for extendability was
not adjusted.  This commit adjusts documentation for interfaces of index AMs
and opclasses.

Discussion: https://postgr.es/m/CAH2-WzmQnW6%2Bz5F9AW%2BSz%2BzEcEvXofTwh_A9J3%3D_WA-FBP0wYg%40mail.gmail.com
Author: Alexander Korotkov
Reported-by: Peter Geoghegan
Reviewed-by: Peter Geoghegan

doc/src/sgml/gin.sgml

@@ -380,7 +380,7 @@
 
  <para>
   Optionally, an operator class for <acronym>GIN</acronym> can supply the
-  following method:
+  following methods:
 
   <variablelist>
     <varlistentry>
@@ -402,6 +402,30 @@
       </para>
      </listitem>
     </varlistentry>
+    <varlistentry>
+     <term><function>void options(local_relopts *relopts)</function></term>
+     <listitem>
+      <para>
+       Defines set of user-visible parameters that control operator class
+       behavior.
+      </para>
+
+      <para>
+       The <function>options</function> function has given pointer to
+       <replaceable>local_relopts</replaceable> struct, which needs to be
+       filled with s set of operator class specific options.  The options
+       can be accessed from other support functions using
+       <literal>PG_HAS_OPCLASS_OPTIONS()</literal> and
+       <literal>PG_GET_OPCLASS_OPTIONS()</literal> macros.
+      </para>
+
+      <para>
+       Since both key extraction for indexed value and representation of the
+       key in <acronym>GIN</acronym> are flexible, it may depends on
+       user-specified parameters.
+      </para>
+     </listitem>
+    </varlistentry>
   </variablelist>
  </para>