Add links to RDoc back into API documentation guidelines [ci skip]

These were removed from #15542 unfortunately :(

guides/source/api_documentation_guidelines.md

@@ -13,7 +13,8 @@ After reading this guide, you will know:
 RDoc
 ----
 
-The [Rails API documentation](http://api.rubyonrails.org) is generated with RDoc.
+The [Rails API documentation](http://api.rubyonrails.org) is generated with
+[RDoc](http://docs.seattlerb.org/rdoc/).
 
 ```bash
   bundle exec rake rdoc
@@ -21,6 +22,11 @@ The [Rails API documentation](http://api.rubyonrails.org) is generated with RDoc
 
 Resulting HTML files can be found in the ./doc/rdoc directory.
 
+Please consult the RDoc documentation for help with the
+[markup](http://docs.seattlerb.org/rdoc/RDoc/Markup.html),
+and also take into account these [additional
+directives](http://docs.seattlerb.org/rdoc/RDoc/Parser/Ruby.html).
+
 Wording
 -------