gh-84747: Add async for comment for StreamReader

[slateny]

https://docs.python.org/dev/library/asyncio-stream.html#asyncio.StreamReader

Perhaps the example under https://docs.python.org/dev/library/asyncio-stream.html#tcp-echo-client-using-streams can be adjusted to look like

    ...
    data = await reader.read(100)
    print(f'Received: {data.decode()!r}')

    async for line in reader:
        print(line)

    print('Close the connection')
    writer.close()
    ...

but it's optional

• Issue: asyncio.StreadReader async for line in reader is not documented #84747

[JelleZijlstra]

I can merge this if @kumaraditya303 is happy.

[miss-islington]

Thanks @slateny for the PR, and @JelleZijlstra for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10, 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-98690 is a backport of this pull request to the 3.11 branch.

[bedevere-bot]

GH-98691 is a backport of this pull request to the 3.10 branch.