Add better warning for nested surrounding forms

As there was some evolutions in 5.7, when migrating a project from 5.6 to 5.8, a new warning appears
"On layout, widget '(no label)': since version 5.7, widgets displayed on the summary panel sometimes contain their own form. This widget should not add another form around itself or the widgets it renders may misbehave."

This warning should be:
"On layout, widget '(no label)': You must uncheck the property "Add surrounding form" as since version 5.7, widgets displayed on the summary panel sometimes contain their own form. This widget should not add another form around itself or the widgets it renders may misbehave."