Give a purgeParserCache invocation that runs

The script takes no bare invocation: without --age or --expiredate it
exits with "Must specify either --expiredate or --age". Passing
--age 0 deletes every object created more than zero seconds ago, and
since CacheTime caps each entry's expiry at $wgParserCacheExpireTime,
that reaches the whole parser cache.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: malberts

docs/migration-guide-bs5.md

@@ -72,7 +72,7 @@ Pages rendered before the upgrade still carry Bootstrap 4 markup in the
 parser cache while the wiki already serves Bootstrap 5 CSS and
 JavaScript, so components on cached pages can look broken. This
 resolves as pages re-render; to force it, purge the affected pages or
-run the `purgeParserCache` maintenance script.
+run `purgeParserCache.php --age 0` to clear the parser cache.
 
 [BS5-jumbotron]: https://getbootstrap.com/docs/5.3/examples/jumbotron/
 [BS5-migration]: https://getbootstrap.com/docs/5.3/migration/