Add comment to BROWSER_LIKE_ACCEPTS [ci skip]

The concept of treating Accept header lists including "*/*" as
identifying the request as coming from a browser, and allowing the mime
negotiation to fall back to a HTML response, was introduced in commits
1310231c15 and dc5300adb6.

I am adding the comment (based on the commit messages of the two commits
mentioned) because it is far from obvious why this regex exists and what
it does.

Co-authored-by: Matthew Riddle <mriddle89@gmail.com>

actionpack/lib/action_dispatch/http/mime_negotiation.rb

@@ -152,6 +152,8 @@ module ActionDispatch
       end
 
       private
+        # We use normal content negotiation unless you include */* in your list,
+        # in which case we assume you're a browser and send HTML.
         BROWSER_LIKE_ACCEPTS = /,\s*\*\/\*|\*\/\*\s*,/
 
         def params_readable? # :doc: