Update docs on request cookies

[RasmusWL]

What do these changes do?

Update docs on request cookies type, so it matches what is actually used in the code

aiohttp/aiohttp/web_request.py

Lines 565 to 573 in 63eb6ff

	@reify
	def cookies(self) -> Mapping[str, str]:
	"""Return request cookies.
	A read-only dictionary-like object.
	"""
	raw = self.headers.get(hdrs.COOKIE, "")
	parsed = SimpleCookie(raw) # type: SimpleCookie[str]
	return MappingProxyType({key: val.value for key, val in parsed.items()})

Are there changes in behavior for the user?

No

Related issue number

No (I searched, but did not find any)

Checklist

• I think the code is well written: N/A
• Unit tests for the changes exist: N/A
• Documentation reflects the changes: N/A
• If you provide code modification, please add yourself to CONTRIBUTORS.txt: N/A
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[RasmusWL]

I did my best on wording, but I'm not normally a documentation writer 😊

[webknjaz]

I'd like to have an intersphinx reference to the actual type in the CPython docs. https://webknjaz.github.io/intersphinx-untangled/docs.python.org should help you construct it.

[RasmusWL]

👍 Should be fixed by 767f6f5

[patchback]

Backport to 3.8: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.8/4f48e3f74549053dda7ea359c34e063660a7ead0/pr-5725

Backported as #5735

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