Add note to `typing.Union` that it is recommended to use `|` instead

[sobolevn]

BPO	46120
Nosy	@gpshead, @asvetlov, @ambv, @miss-islington, @sobolevn, @Fidget-Spinner
PRs	bpo-46120: State that | is preferred over Union #30222 [3.10] bpo-46120: State that | is preferred over Union (GH-30222) #30250

^{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 2021-12-29.11:57:14.805>
created_at = <Date 2021-12-18.08:20:39.434>
labels = ['3.11', 'type-bug', '3.10', 'docs']
title = 'Add note to `typing.Union` that it is recommended to use `|` instead'
updated_at = <Date 2021-12-29.21:33:41.569>
user = 'https://github.com/sobolevn'

bugs.python.org fields:

activity = <Date 2021-12-29.21:33:41.569>
actor = 'sobolevn'
assignee = 'docs@python'
closed = True
closed_date = <Date 2021-12-29.11:57:14.805>
closer = 'AlexWaygood'
components = ['Documentation']
creation = <Date 2021-12-18.08:20:39.434>
creator = 'sobolevn'
dependencies = []
files = []
hgrepos = []
issue_num = 46120
keywords = ['patch']
message_count = 5.0
messages = ['408836', '409158', '409302', '409316', '409328']
nosy_count = 7.0
nosy_names = ['gregory.p.smith', 'asvetlov', 'docs@python', 'lukasz.langa', 'miss-islington', 'sobolevn', 'kj']
pr_nums = ['30222', '30250']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46120'
versions = ['Python 3.10', 'Python 3.11']

[sobolevn]

As discussed in https://mail.python.org/archives/list/typing-sig@python.org/thread/TW5M6XDN7DO346RXMADKBAAGNSZPC4ZQ/

we could probably stick a huge notice in the typing.Union docs saying | is preferred for readability in most cases, and not deprecate it for now.

I will send a PR to Union and Optional docs with this note today.

[gpshead]

New changeset 1b30660 by Nikita Sobolev in branch 'main':
bpo-46120: State that | is preferred over Union (GH-30222)
1b30660

[miss-islington]

New changeset ed1671c by Miss Islington (bot) in branch '3.10':
[3.10] bpo-46120: State that | is preferred over Union (GH-30222) (GH-30250)
ed1671c