Add recommendation/guidance about not sharing outlets between modals and/or modal dialogs

[notmessenger]

I had a situation where the closing of one modal was displaying a new modal dialog, who were both using the same frost-modal-outlet. The size of the previously-displayed content was causing the new modal dialog to be displayed with a lot of blank space, causing the need to scroll up to see the content, which when displayed would cause the scrollbar to disappear.

This was due to the shared frost-modal-outlet and the previous content size(ing) being used for the new display. Giving each modal (dialog) their own outlet resolved this.