doc: gin_page_opaque_info() must be a _compressed_ GIN page

Reported-by: Jeff Janes

Discussion: https://postgr.es/m/CAMkU=1xvzQxTAiYNM2PWJ6snMTPh3u3Ammbwss7mvAShS2Ohww@mail.gmail.com

Author: Jeff Janes

Backpatch-through: master

doc/src/sgml/pageinspect.sgml

@@ -700,7 +700,7 @@ test=# SELECT * FROM gin_page_opaque_info(get_raw_page('gin_index', 2));
     <listitem>
      <para>
       <function>gin_leafpage_items</function> returns information about
-      the data stored in a <acronym>GIN</acronym> leaf page.  For example:
+      the data stored in a compressed <acronym>GIN</acronym> leaf page.  For example:
 <screen>
 test=# SELECT first_tid, nbytes, tids[0:5] AS some_tids
         FROM gin_leafpage_items(get_raw_page('gin_test_idx', 2));