Print warning message when using Prism Ruby parser

Author: st0012

lib/rdoc/parser/ruby.rb

@@ -11,6 +11,9 @@
 if ENV['RDOC_USE_PRISM_PARSER']
   require 'rdoc/parser/prism_ruby'
   RDoc::Parser.const_set(:Ruby, RDoc::Parser::PrismRuby)
+  puts "========================================================================="
+  puts "RDoc is using the experimental Prism parser to generate the documentation"
+  puts "========================================================================="
   return
 end