Better check for brackets in field names (in form helper)

This fixes a bug if your checkbox or select field name is only 2 characters long (which would erroneously make the helper functions think you had brackets in the field name and hence output the wrong id).

This will also fix a hypothetical situation where brackets appear in a field name more than once (not sure why someone would want to do that though).

I also fixed an incorrect comment on the select function.