Doc async hooks typo

[Rawal27]

## Description

Update the `async_hooks` documentation to improve grammar, punctuation, and consistency.

### Changes

- Correct grammatical errors and subject-verb agreement.
- Improve sentence structure and readability.
- Add missing punctuation where appropriate.
- Use consistent Markdown formatting for API names and callbacks.
- Improve compound-word formatting, such as `continuation-local`.
- Clarify a few sentences without changing their technical meaning.

These changes are documentation-only and do not affect any runtime behavior or APIs.