Add useful comment.

lib/jekyll/url.rb

@@ -84,6 +84,12 @@ module Jekyll
       end
     end
 
+    # We include underscores in keys to allow for 'i_month' and so forth.
+    # This poses a problem for keys which are followed by and underscore
+    # but the underscore is not part of the key, e.g. '/:month_:day'.
+    # That should be :month and :day, but our key extraction regexp isn't
+    # smart enough to know that so we have to make it an explicit
+    # possibility.
     def possible_keys(key)
       if key.end_with?("_")
         [key, key.chomp("_")]