[docs] Update behavior for user-defined awaiter

### Search before asking

- [x] I searched the [issues](https://github.com/alibaba/async_simple/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!

Start with the user-defined awaiter behavior described in the issue and discussion #162, then locate the existing documentation for awaiters and the ViaCoroutine helper. Done when the documentation explains what happens when an awaiter lacks a coAwait method and how context switching behaves.

Written by the indexing model from the issue text.