A somewhat separate issue is the lack of even a mention of the existence
of the Canvas widget, which would merit a separate subsection if
documented. I think it should at least be mentioned with a reference to
the online reference materials listed before.

Indeed, the tkinter reference should somewhere have a complete list of
widgets, just like the ttk section does

24.2.2. Ttk Widgets¶

Ttk comes with 17 widgets, eleven of which already existed in tkinter:
Button, Checkbutton, Entry, Frame, Label, LabelFrame, Menubutton,
PanedWindow, Radiobutton, Scale and Scrollbar. The other six are new:
Combobox, Notebook, Progressbar, Separator, Sizegrip and Treeview. And
all them are subclasses of Widget.