Deprecated ::draw signal is in the getting started markdown file
The following is extracted from "Custom Drawing" section in docs/reference/gtk/getting_started.md There's description about ::draw signal, but it is replaced by snapshot function in gtk4. So, the description needs to be modified and updated.
Many widgets, like buttons, do all their drawing themselves. You just tell them the label you want to see, and they figure out what font to use, draw the button outline and focus rectangle, etc. Sometimes, it is necessary to do some custom drawing. In that case, a GtkDrawingArea might be the right widget to use. It offers a canvas on which you can draw by connecting to the ::draw signal. The contents of a widget often need to be partially or fully redrawn, e.g. when another window is moved and uncovers part of the widget, or when the window containing it is resized. It is also possible to explicitly cause part or all of the widget to be redrawn, by calling gtk_widget_queue_draw() or its variants. GTK takes care of most of the details by providing a ready-to-use cairo context to the ::draw signal handler. The following example shows a ::draw signal handler. It is a bit more complicated than the previous examples, since it also demonstrates input event handling by means of event controllers.