[docs] Update behavior for user-defined awaiter

[ChuanqiXu9]

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• [x] I searched the issues and found no similar issues.

What happened + What you expected to happen

When user defines an awaiter, if the awaiter didn't implement coAwait method, the ViaCoroutine helper will be invoked to switch the context. However, the behavior is not documented. So here is the confusion: https://github.com/alibaba/async_simple/discussions/162

• [x] Yes I am willing to submit a PR!