[ruby/reline] Add URL reference of history-size

https://github.com/ruby/reline/commit/13420197b8

lib/reline/config.rb

@@ -52,6 +52,7 @@ class Reline::Config
     @key_actors[:emacs] = Reline::KeyActor::Emacs.new
     @key_actors[:vi_insert] = Reline::KeyActor::ViInsert.new
     @key_actors[:vi_command] = Reline::KeyActor::ViCommand.new
+    # https://tiswww.case.edu/php/chet/readline/readline.html#IDX25
     @history_size = -1 # unlimited
     @keyseq_timeout = 500
     @test_mode = false