doc: add MultiPool & MultiPoolFunc example code and update READMEs.

[POABOB]

1. Are you opening this pull request for bug-fixs, optimizations or new feature?

Update example code and documentation for the newly released feature.

2. Please describe how these code changes achieve your intention.

There is no example for MultiPool & MultiPoolFunc.

3. Please link to the relevant issues (if any).

No.

4. Which documentation changes (if any) need to be made/updated because of this PR?

No.

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[panjf2000]

LGTM

[panjf2000]

Thanks~