doc:  mention  pg_upgrade extension script
Since commit pg_upgrade automatically creates a script to
update extensions, so mention that instead of ALTER EXTENSION.
Backpatch-through: 9.6
@@ -308,8 +308,9 @@ make prefix=/usr/local/pgsql.new install
must be installed in the new cluster, usually via operating system
commands. Do not load the schema definitions, e.g., <command>CREATE
EXTENSION pgcrypto</command>, because these will be duplicated from
- the old cluster. (Extensions with available updates can be processed
- later using <literal>ALTER EXTENSION ... UPDATE</literal>.)
+ the old cluster. If extension updates are available,
+ <application>pg_upgrade</application> will report this and create
+ a script that can be run later to update them.
</para>
</step>
