gh-98641: Task group vs gather

[why-not-try-calmer]

The purpose of this PR is to clarify the difference(s) between TaskGroup and gather. There are two possibilities:

• the usecases of two merely overlap: then this PR should clarify the differences so that the user knows which is the better fit for the situation (the comments made here go somewhat in that direction);
• the use-cases do coincide or one is included in the other: then this PR should explain why gather has not been deprecated yet (maintaining it for compatibility reasons is fine, but this PR should make it explicit).

The end result should better reflect the point of maintaining the two approaches side by side in Python for the years to come, as it has significant impact on the habits that people might develop when writing concurrent Python.

I apologize for not responding to the events that lead to the closing of the Issue referenced below, I was busy on other things. Having my PR + accompanying Issue closed were a good wake-up call.

• Issue: The documentation about asyncio.TaskGroup might portray it as a replacement for asyncio.gather #98641

[gvanrossum]

[...] asyncio.gather() works well for tasks that do not schedule tasks themselves, especially when the caller needs to consume their result as soon as possible.

I disagree with this part of your proposed commit message, so I removed it. I don't see any mention of this opinion in the changes themselves to I merged your PR. Thanks for your contribution! The docs are better now.

[why-not-try-calmer]

[...] asyncio.gather() works well for tasks that do not schedule tasks themselves, especially when the caller needs to consume their result as soon as possible.

I disagree with this part of your proposed commit message, so I removed it. I don't see any mention of this opinion in the changes themselves to I merged your PR. Thanks for your contribution! The docs are better now.

Thanks. For my education, what's the status of gather() in the grand scheme of things? Is it going to be maintained for compatibility until deprecation? Because the phrasing of docs -- and my PR didn't affect that -- suggests that the use-case for gather(), beyond mere syntactical convenience, is dim.

[gvanrossum]

I have no desire to deprecate gather().