Update documentation for upgrading PHP

** Why are these changes being introduced:

* The existing how-to documentation for upgrading PHP versions does not
  address rebuilding the local Lando environment.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/pw-11

** How does this address that need:

* This adds an extra step to the how-to document, detailing the Lando
  command needed, and when it should be run.

** Document any side effects to this change:

* None

docs/howto/upgrading-php.md

@@ -47,7 +47,14 @@ consult the Site Health report (WordPress admin interface -> Tools -> Site
 Health). The report's Info tab includes a Server panel which reports the PHP
 version.
 
-4. Push branch to Github for code review
+4. Rebuild your local Lando environment with these changes
+
+Running `lando rebuild` will apply any needed container updates, now that the
+new PHP version is specified in `pantheon.upstream.yml`. The output of this
+command should include confirmation of the new PHP version, as the relevant
+pre-install command will be triggered during this process.
+
+5. Push branch to Github for code review
 
 From this point, follow our standing practices for pull requests and code
 review.