[issue37937] Mention ``frame.f_trace`` in :func:`sys.settrace` docs.

[Serhiy Storchaka]

New submission from Serhiy Storchaka <storchaka+cpython at gmail.com>:

AFAIK setting frame.f_trace *instead* of calling sys.settrace() does not work. You need to call sys.settrace() to enable tracing.

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nosy: +serhiy.storchaka

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37937>
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