Adjust hints and docs to suggest CREATE EXTENSION not CREATE LANGUAGE.

The core PLs have been extension-ified for seven years now, and we can
reasonably hope that all out-of-core PLs have been too.  So adjust a few
places that were still recommending CREATE LANGUAGE as the user-level
way to install a PL.

Discussion: https://postgr.es/m/CA+TgmoaJTUDMSuSCg4k08Dv8vhbrJq9nP3ZfPbmysVz_616qxw@mail.gmail.com

doc/src/sgml/ref/create_transform.sgml

@@ -157,7 +157,7 @@ CREATE [ OR REPLACE ] TRANSFORM FOR <replaceable>type_name</replaceable> LANGUAG
 <programlisting>
 CREATE TYPE hstore ...;
 
-CREATE LANGUAGE plpythonu ...;
+CREATE EXTENSION plpythonu;
 </programlisting>
    Then create the necessary functions:
 <programlisting>
@@ -176,7 +176,7 @@ CREATE TRANSFORM FOR hstore LANGUAGE plpythonu (
     TO SQL WITH FUNCTION plpython_to_hstore(internal)
 );
 </programlisting>
-   In practice, these commands would be wrapped up in extensions.
+   In practice, these commands would be wrapped up in an extension.
   </para>
 
   <para>

doc/src/sgml/ref/do.sgml

@@ -81,7 +81,7 @@ DO [ LANGUAGE <replaceable class="parameter">lang_name</replaceable> ] <replacea
 
   <para>
    The procedural language to be used must already have been installed
-   into the current database by means of <command>CREATE LANGUAGE</command>.
+   into the current database by means of <command>CREATE EXTENSION</command>.
    <literal>plpgsql</literal> is installed by default, but other languages are not.
   </para>
 

src/backend/commands/functioncmds.c

@@ -934,7 +934,7 @@ CreateFunction(ParseState *pstate, CreateFunctionStmt *stmt)
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("language \"%s\" does not exist", language),
 				 (PLTemplateExists(language) ?
-				  errhint("Use CREATE LANGUAGE to load the language into the database.") : 0)));
+				  errhint("Use CREATE EXTENSION to load the language into the database.") : 0)));
 
 	languageOid = HeapTupleGetOid(languageTuple);
 	languageStruct = (Form_pg_language) GETSTRUCT(languageTuple);
@@ -2136,7 +2136,7 @@ ExecuteDoStmt(DoStmt *stmt, bool atomic)
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("language \"%s\" does not exist", language),
 				 (PLTemplateExists(language) ?
-				  errhint("Use CREATE LANGUAGE to load the language into the database.") : 0)));
+				  errhint("Use CREATE EXTENSION to load the language into the database.") : 0)));
 
 	codeblock->langOid = HeapTupleGetOid(languageTuple);
 	languageStruct = (Form_pg_language) GETSTRUCT(languageTuple);