Show warning message if delete_all is firing callbacks

`post.comments.delete_all` will fire callbacks if :dependent option is
:destroy . It will be fixed in Rails 4.1 . In the meantime display
a warning . Look at #9567 for details .
Merge pull request #9668 from neerajdotname/9567-delete-all-warning

Show warning message if delete_all is firing callbacks
documents how to DROP DEFAULT [ci skip]
quick pass over the RDoc of schema_statements.rb [ci skip]
promotes change_column_null to the migrations API