The phrasing "corner of the screen" wouldn't be accurate when the user
is doing a split screen or if the window or app is not taking up the
whole screen. Also, the use of "lower/upper" and "top/bottom" is not
consistent.
This standardizes on "corner of the app" and "bottom/top" when
referring to the location of app features on Desktop, Web, or Mobile.
Updates help center article on viewing and editing drafts of
messages for current documentation styles (less use of bold text,
no Mac keys, tabs for Desktop/Web vs subheaders, etc), which also
preps page for mobile app documentation when drafts are supported.