[ruby/reline] Add comment for unused constants

FILENAME_COMPLETION_PROC and USERNAME_COMPLETION_PROC are not used by Reline.
However, they were added for compatibility with the rb-readline gem.
These constants have been retained and comments added.

https://github.com/ruby/reline/commit/98fdbd3f18

lib/reline.rb

@@ -11,6 +11,7 @@ require 'reline/terminfo'
 require 'rbconfig'
 
 module Reline
+  # NOTE: For making compatible with the rb-readline gem
   FILENAME_COMPLETION_PROC = nil
   USERNAME_COMPLETION_PROC = nil