Document that querystring values are not unquoted twice

[winklemad]

Description of change

urllib.parse.parse_qs already percent-decodes query values. Both the sync and
async URL parsers subsequently called unquote on those values, causing
doubly encoded data such as %2520 to become a space instead of the literal
%20.

Remove the redundant second decoding step in both implementations and add
mirrored sync/async regression tests proving query values are decoded exactly
once.

Fixes #4208

Pull Request check-list

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---

Note

Low Risk
Doc-only changes to public API descriptions; no runtime behavior in the shown diff.

Overview
Updates from_url / ConnectionPool.from_url docstrings in sync and async client, cluster, and connection modules so they match actual URL parsing behavior.

The docs no longer say querystring values are passed through urllib.parse.unquote. They now state that username, password, hostname, and path are unquoted, while query parameters are decoded once by parse_qs and are not unquoted again—avoiding the misleading impression of double-decoding (e.g. %2520 becoming a space instead of %20, per #4208).

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[Sanjays2402]

the from_url docstrings still promise the old behaviour: "The username, password, hostname, path and all querystring values are passed through urllib.parse.unquote" (client.py:194, connection.py:2938, cluster.py:669, plus the three asyncio twins). querystring values are the one item in that list that no longer goes through unquote after this, so it should probably be dropped from the sentence.

[winklemad]

You're right, and thanks for the precise pointers — I'd missed that the sentence wraps across two lines.

Since #4222 landed the actual fix on master, I've narrowed this PR down to just the docs half, which is the part still outstanding. All six from_url docstrings now read that username/password/hostname/path go through unquote, and that querystring values are decoded by parse_qs and not unquoted again. No code or test changes left — those are upstream's now, so the branch is conflict-free against master again.

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[petyaslavova]

Hey @winklemad, thank you for your contribution! The change LGTM.