How to reproduce and improve : 

1/ Create an article and select "Form" menu => CMS Form tab is opened. 

2/ CMS Form tab is opened. Click on "Acknowledgment". 

3/ A pop-up is opened with following information (french and english) : 

=> For french message, is it possible to precise : 

• Champ texte simple
• utilisant une règle "email"
• La propriété règle est un champ disponible dans l'onglet "Texte simple" 

=> for english message, is it possible to precise : 

• must contains a "simple text" field. 
• Property rule is available in tab "Simple Text".