CellEditable: Clarify doc on lifecycle of editable
* Note in the intro that we're really thinking about temporary widgets
* Mention a gotcha regarding GtkEntry and how ::focus-out stops editing
* Give some examples of what you'd want to do in ::editing-done
* Be a bit more precise about what ::remove-widget represents
* Summarise the lifecycle between Renderer/Editable in .start_editing()
* Emphasise again there that this should be viewed as a temporary widget
https://gitlab.gnome.org/GNOME/gtk/issues/154