The preview button on the page edit bar should show a tooltip explaining both what it does and hinting that you can also hit the ESC key in order to switch between editing a page and previewing it.

Please use the following:

• when editing a page: "Preview page [ESC]"
• when showing a page preview: "Edit page [ESC]"

There need not be any tooltips on the menu entries allowing to choose the type of preview (e.g. "Desktop" or "Smartphone"). Just show the tooltip on hover on the button itself.