[DOC] Use the recent RDoc always to generate the doc of ruby itself

tool/rdoc-srcdir

@@ -1,5 +1,6 @@
-#!ruby
+#!ruby -W0
 
+$:.unshift(File.expand_path("../lib", __dir__))
 require 'rdoc/rdoc'
 
 # Make only the output directory relative to the invoked directory.