Add compatibility information for prepared transaction commands

doc/src/sgml/ref/commit_prepared.sgml

@@ -83,6 +83,18 @@ COMMIT PREPARED 'foobar';
 
  </refsect1>
 
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   <command>COMMIT PREPARED</command> is a
+   <productname>PostgreSQL</productname> extension.  It is intended for use by
+   external transaction management systems, some of which are covered by
+   standards (such as X/Open XA), but the SQL side of those systems is not
+   standardized.
+  </para>
+ </refsect1>
+
  <refsect1>
   <title>See Also</title>
 

doc/src/sgml/ref/prepare_transaction.sgml

@@ -156,6 +156,18 @@ PREPARE TRANSACTION 'foobar';
 </programlisting></para>
  </refsect1>
 
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   <command>PREPARE TRANSACTION</command> is a
+   <productname>PostgreSQL</productname> extension.  It is intended for use by
+   external transaction management systems, some of which are covered by
+   standards (such as X/Open XA), but the SQL side of those systems is not
+   standardized.
+  </para>
+ </refsect1>
+
  <refsect1>
   <title>See Also</title>
 

doc/src/sgml/ref/rollback_prepared.sgml

@@ -83,6 +83,18 @@ ROLLBACK PREPARED 'foobar';
 
  </refsect1>
 
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   <command>ROLLBACK PREPARED</command> is a
+   <productname>PostgreSQL</productname> extension.  It is intended for use by
+   external transaction management systems, some of which are covered by
+   standards (such as X/Open XA), but the SQL side of those systems is not
+   standardized.
+  </para>
+ </refsect1>
+
  <refsect1>
   <title>See Also</title>