fix base_url param documentation

[alexis974]

What do these changes do?

This PR update the documentation made in 38ffe6f to notify about a behavior introduce in 527b1b9

base_url must not include path, otherwise an AssertionError is raised

File "/XXXXXXXXXXXXXXXXXX/aiohttp/client.py", line 248, in __init__
    self._base_url.origin() == self._base_url
AssertionError: Only absolute URLs without path part are supported

Are there changes in behavior for the user?

No changes in behavior for the user since this is a documentation update

Is it a substantial burden for the maintainers to support this?

No

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES/ folder

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.53%. Comparing base (6c3122f) to head (b515b76).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8197   +/-   ##
=======================================
  Coverage   97.53%   97.53%           
=======================================
  Files         107      107           
  Lines       32843    32843           
  Branches     3851     3851           
=======================================
  Hits        32032    32032           
  Misses        610      610           
  Partials      201      201           

Flag	Coverage Δ
CI-GHA	97.44% <ø> (ø)
OS-Linux	97.11% <ø> (ø)
OS-Windows	95.62% <ø> (ø)
OS-macOS	96.66% <ø> (-0.27%)	⬇️
Py-3.10.11	95.54% <ø> (ø)
Py-3.10.13	96.92% <ø> (ø)
Py-3.11.8	96.56% <ø> (-0.04%)	⬇️
Py-3.12.2	96.68% <ø> (ø)
Py-3.8.10	95.52% <ø> (ø)
Py-3.8.18	96.85% <ø> (ø)
Py-3.9.13	95.51% <ø> (ø)
Py-3.9.18	96.85% <ø> (-0.03%)	⬇️
Py-pypy7.3.15	96.42% <ø> (ø)
VM-macos	96.66% <ø> (-0.27%)	⬇️
VM-ubuntu	97.11% <ø> (ø)
VM-windows	95.62% <ø> (ø)

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[patchback]

Backport to 3.9: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.9/0e91eb0bd94298e501084f245df44cc1772c49e5/pr-8197

Backported as #8198

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.10: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.10/0e91eb0bd94298e501084f245df44cc1772c49e5/pr-8197

Backported as #8199

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.