bpo-37937: Mention frame.f_trace in sys.settrace docs (GH-15439)


Mention frame.f_trace in sys.settrace docs, as well as the fact you still
need to call `sys.settrace` to enable the tracing machinery before setting
`frame.f_trace` will have any effect.
(cherry picked from commit 9c2682efc69568e1b42a0c1759489d6f2e3b30ea)

Co-authored-by: Ram Rachum <ram@rachum.com>
2 files changed