[improve][site] Update description for producer's SendAsync method

[jasonyunicorn]

✅ Contribution Checklist

• I read the contribution guide
• I updated the versioned docs

This PR addresses apache/pulsar#23643

[jasonyunicorn]

Here is a screenshot of things running locally with the updated description:

[lhotari]

Thanks for the PR.
We currently maintain docs for 4.0.x, 3.3.x and 3.0.x. Usually we don't make updates to archived versions. However, since in this case, there isn't a matching documentation in maintained versions, I don't really know where it is available for the Go client. Could someone working on the Pulsar Go client help with this? /cc @crossoverJie @nodece @RobertIndie @BewareMyPower