Fix some incorrect async functions, improve frontend document.

[wxiaoguang]

As discussed in #17448 , this PR fixes some incorrect async functions, and improve frontend document about async functions.

Reference: https://github.com/github/eslint-plugin-github/blob/main/docs/rules/async-preventdefault.md

(In future we may use this plugin, but not now)

[Gusted]

Is there any specific reason to have code like const _promise =, as _promise will be a unused variable.

[wxiaoguang]

Is there any specific reason to have code like const _promise =, as _promise will be a unused variable.

Yep, there are reasons since we call an async in a non-async context.

1. Tell the readers that this is done by purpose, we want to call the async function and ignore the Promise.
2. Some lint rules or IDEs report warnings about "returned Promise is ignored", so this method can make them happy.