doc: Improve async_hooks documentation wording

[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.

[aduh95]

Please stop opening PRs faster they can be reviewed. If you do not stop, you will get blocked from the organisation.

[Rawal27]

@aduh95 Can you clarify this? I mean, these are genuine doc-level changes that I have proposed. Also, a contributor can contribute to as many as 30 pull requests at a time. Closing them without reason seems unfair to me.