Add or tweak documentation for the controllers

Small improvements and additions to the documentation of the Scrapbook
controllers.

app/controllers/scrapbook/application_controller.rb

@@ -4,7 +4,6 @@ module Scrapbook
   # @private
   #
   # Base controller for collecting shared functionality for the Scaapbook App.
-  # placeholder
   class ApplicationController < ActionController::Base
   end
 end

app/controllers/scrapbook/empty_controller.rb

@@ -2,6 +2,14 @@
 
 module Scrapbook
   # This controller is used to specify view paths to check if a template exists.
+  #
+  # Scapbook views (Rails templates) exist outside the normal Rails views hierarchy. This is
+  # fine, we can prepend the view path for Scrapbook pages to our controller when we need to
+  # render Scrapbook pages. However, part of the process for to determine if we need to
+  # render a Scrapbook page is determining if the template exists. We want to know if the
+  # template exists before we prepend the path to the controller and render it. This empty
+  # controller allows us to call `EmptyController#template_exists?` to do just that after we
+  # prepend the view path to it.
   class EmptyController < ApplicationController
     self.view_paths = []
   end

app/controllers/scrapbook/pages_controller.rb

@@ -2,11 +2,17 @@
 
 module Scrapbook
   # @todo Document this controller
+  # Handles requests for specific pages — either a full page load, or Turbo Drive request
+  # for navigation.
+  # Handles requests to display a specifc page — both for the Scrapbook application itself
+  # and the content within the user's Scrapbook.
   class PagesController < ApplicationController
     self.view_paths = Engine.config.paths['app/views'].to_a
 
     layout -> { false if request.headers.include?('Turbo-Frame') }
 
+    # Displays the requested page for both full page loads, or Turbo Drive requests from
+    # page navigation.
     def show
       return head(:not_found) if (scrapbook = find_scrapbook).nil?
 
@@ -20,6 +26,9 @@ def show
       end
     end
 
+    # Handles translating the path to look up and generate the content from the Scrapbook.
+    # The content could be a template page, a template for a directory, the default content
+    # for directories without associated content pages, or just a raw file (like an image).
     def raw
       return head(:not_found) if (scrapbook = find_scrapbook).nil?