The return type for `_make_custom_tooltip` is clarified as Control, and users
should make sure to return a visible node for proper size calculations.
Moreover in the current master branch, a PopupPanel will be added as parent
to the provided tooltip to make it a sub-window.
Clarifies documentation for `Control._make_custom_tooltip`, and shows how to
use the (until now undocumented) "TooltipPanel" and "TooltipLabel" theme types
to style tooltips.
Fixes#39677.
c5d8dafec4