Add more explicit guidance on how to fix callback deprecations

This deprecation warning message will be more useful if it indicates what the string was doing — being eval'd — and what the non-deprecated options for callback conditionals are.

activesupport/lib/active_support/callbacks.rb

@@ -662,8 +662,10 @@ def set_callback(name, *filter_list, &block)
 
           if options[:if].is_a?(String) || options[:unless].is_a?(String)
             ActiveSupport::Deprecation.warn(<<-MSG.squish)
-              Passing string to :if and :unless conditional options is deprecated
-              and will be removed in Rails 5.2 without replacement.
+              Passing string to be evaluated in :if and :unless conditional
+              options is deprecated and will be removed in Rails 5.2 without
+              replacement. Pass a symbol for an instance method, or a lamdba,
+              proc or block, instead.
             MSG
           end