Document the "gotcha" behaviors in utcnow() and utcfromtimestamp()

BPO	37488
Nosy	@abalkin, @pganssle, @miss-islington, @epicfaace
PRs	bpo-37488 : Document a warning for datetime.utcnow() and utcfromtimestamp() #15773 [3.8] bpo-37488 : Document a warning for datetime.utcnow() and utcfromtimestamp() (GH-15773) #16059

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-09-12.14:59:27.100>
created_at = <Date 2019-07-02.16:01:38.841>
labels = ['3.8', 'type-feature', '3.7', '3.9', 'docs']
title = 'Document the "gotcha" behaviors in utcnow() and utcfromtimestamp()'
updated_at = <Date 2019-09-12.14:59:27.100>
user = 'https://github.com/pganssle'

bugs.python.org fields:

activity = <Date 2019-09-12.14:59:27.100>
actor = 'p-ganssle'
assignee = 'docs@python'
closed = True
closed_date = <Date 2019-09-12.14:59:27.100>
closer = 'p-ganssle'
components = ['Documentation']
creation = <Date 2019-07-02.16:01:38.841>
creator = 'p-ganssle'
dependencies = []
files = []
hgrepos = []
issue_num = 37488
keywords = ['patch']
message_count = 5.0
messages = ['347148', '351567', '351884', '352189', '352192']
nosy_count = 5.0
nosy_names = ['belopolsky', 'docs@python', 'p-ganssle', 'miss-islington', 'epicfaace']
pr_nums = ['15773', '16059']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37488'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']