Update documentation to reflect that standard PLs are now extensions.

Recommend use of CREATE EXTENSION rather than plain CREATE LANGUAGE
where relevant.  Encourage PL authors to provide extension wrappers
for their PLs.

doc/src/sgml/plhandler.sgml

@@ -216,6 +216,15 @@ CREATE LANGUAGE plsample
     should execute this code and return.
    </para>
 
+   <para>
+    It's recommended that you wrap all these function declarations,
+    as well as the <command>CREATE LANGUAGE</> command itself, into
+    an <firstterm>extension</> so that a simple <command>CREATE EXTENSION</>
+    command is sufficient to install the language.  See
+    <xref linkend="extend-extensions"> for information about writing
+    extensions.
+   </para>
+
    <para>
     The procedural languages included in the standard distribution
     are good references when trying to write your own language handler.