docs add more docs to app/{jobs,logical}.

These were missed in the last commit.

app/mailers/application_mailer.rb

@@ -1,3 +1,6 @@
+# The base class for emails sent by Danbooru.
+#
+# @see https://guides.rubyonrails.org/action_mailer_basics.html
 class ApplicationMailer < ActionMailer::Base
   default from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.contact_email}>", content_type: "text/html"
 end