There's out-of-date documentation at
- https://www.mediawiki.org/wiki/Wikimedia_Foundation_Design/Patterns_and_components#Flyouts
- https://www.mediawiki.org/wiki/Wikimedia_Foundation_Design/Agora_Control_Library#Flyout
- https://www.mediawiki.org/wiki/Wikimedia_Foundation_Design/Agora_Control_Library/Behavior#Flyout_.2F_popover
but currently there are minimal details at http://livingstyleguide.wmflabs.org/wiki/Dialogs (I think this is where the element type should be documented??)
Our own English article is at https://en.wikipedia.org/wiki/Popover_%28GUI%29 (and much google searching has now confused me as to what a "flyout" really means...)
See also: T88630: Implement a way to display tooltips (using PopupWidget) on hover/focus rather than on click for some discussion about good/bad practices.