Move context manager usage of `contextvars.Token` to `contextvars.ContextVar.set`

[johnslavik]

Documentation

Follow-up to #129889.

The idea behind making contextvars.Token support context manager protocol was to eliminate the need for manipulating the token manually and instead use contextvars.ContextVar.set() directly. Therefore, it is more logical to present the context manager usage example in contextvars.ContextVar.set, not in contextvars.Token.

[johnslavik]

CC @ZeroIntensity mentorship, labels: 3.14, 3.15. (I couldn't find a relevant topic label).

[johnslavik]

In the contextvars.Token documentation, we could instead expand on the behavior and denote that the token cannot be re-entered:

tok = cv.set("foo")

with tok:
    do_this()

do_that()

# this is illegal!
# fails with RuntimeError: <Token used var=<ContextVar name='whatever' ...> has already been used once
with tok:
    do_other_stuff()

For a moment, I thought this was possible.