docs: add docs signed_url expiration take default utc

[HemangChothani]

Fixes #244

[tseaver]

Please change all the docstrings as noted in my first comment.

[tseaver]

The default value of the tzinfo argument to datetime.datetime is None, resulting in a "naive" datetime instance: the comment should state clearly that we forcibly convert such instances to UTC. E.g.:

    :param expiration: Point in time when the signed URL should expire. If
                       a `datetime` instance is passed without an explicit
                      `tzinfo` set,  it will be converted to ``UTC``.

[andrewsg]

Rather than "it will be converted to" let's use "it will be assumed to be". This is a huge footgun; a user who sees the term "converted" will assume that we will do the right thing in the default case, whereas in fact we will almost always do the wrong thing.

[andrewsg]

As per the original issue, can we also modify the system test TestV4POSTPolicies.test_get_signed_policy_v4() to use utcnow()?

[andrewsg]

Cancel that, I just saw that the test issue is resolved in #251. Thanks for that! Leaving review on "changes requested" for the docstring update as per my comment; lgtm other than that.

[andrewsg]

Thanks!

[tseaver]

@HemangChothani I would merge my suggestions to address the concern expressed by @andrewsg , but you have the "maintainers can push changes" option turned off.