[DOC] Fix warn_missing_rdoc_ref default value in doc (#1408)

It's actually enabled by default now. Also removes the setting from
`.rdoc_options` as it's not needed.

Author: st0012

.rdoc_options

@@ -1,6 +1,5 @@
 op_dir: _site # for GitHub Pages and should match the config of RDoc task in Rakefile
 title: rdoc Documentation
 main_page: README.rdoc
-warn_missing_rdoc_ref: true
 autolink_excluded_words:
 - RDoc

lib/rdoc/options.rb

@@ -327,7 +327,7 @@ class RDoc::Options
 
   ##
   # Warn if rdoc-ref links can't be resolved
-  # Default is +false+
+  # Default is +true+
 
   attr_accessor :warn_missing_rdoc_ref