bpo-46480: rephrase typing.assert_type docs (GH-32069)

The goal here is to reduce potential confusion between
`assert_type(val, type)` and `assert isinstance(val, typ)`.

The former is meant to ask a type checker to confirm a fact, the latter
is meant to tell a type checker a fact. The behaviour of the latter more
closely resembles what I'd expect from the prior phrasing of
"assert [something] to the type checker".