Add more gem docs in Rails Application Templates guide

Added doc on how to add a gem with a specific version. Added doc on how to add a comment.

guides/source/rails_application_templates.md

@@ -62,6 +62,18 @@ gem "nokogiri"
 
 Note that this method only adds the gem to the `Gemfile`; it does not install the gem.
 
+You can also specify an exact version:
+
+```ruby
+gem "nokogiri", "~> 1.16.4"
+```
+
+And you can also add comments that will be added to the `Gemfile`:
+
+```ruby
+gem "nokogiri", "~> 1.16.4", comment: "Add the nokogiri gem for XML parsing"
+```
+
 ### gem_group(*names, &block)
 
 Wraps gem entries inside a group.