Improve understanding for 'view as 'name of user'' banner

[lusebille]

We've got a feedback from user(owner) who thinked asses rules were allowed for editors due to 'view as 'Name of user' mode.
Indeed, when the owner uses this mode, it's applied to the entire interface except for the header elements, which can be confusing for the user. we should probably mention this in the banner >> not the good wording but something like : " Notice the header doesn't change it's still your session "

[lusebille]

Mock is here, finally, we're going with an 'info' icon button displaying the message : ' Everything below this message is the preview oh what the user will see'

[jr-grist]

@lusebille Recommendation for language on that message: "The preview below this header shows how the selected user will see this document"