Updating .yardopts to document .rb files in [GEM]/app

Fixes rails#36321

Enables missing documentation for:
- actionmailbox/app/controllers/{.,**}/*.rb
- actionmailbox/app/jobs/{.,**}/*.rb
- actionmailbox/app/models/{.,**}/*.rb
- actiontext/app/models/{.,**}/*.rb
- activestorage/app/controllers/{.,**}/*.rb
- activestorage/app/jobs/{.,**}/*.rb
- activestorage/app/models/{.,**}/*.rb

.yardopts

@@ -1,4 +1,5 @@
 --exclude /templates/
 --quiet
 act*/lib/**/*.rb
+act*/app/**/*.rb
 railties/lib/**/*.rb