style: improve documentation for async ports

[hasezoey]

ISSUE NUMBER - PULL_REQUEST_TITLE

fixes #107

Description

Document how the ports expect events to be returned and how the function should behave.

Type of change

Please select relevant options.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the contribution guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I formatted the code with cargo fmt
• I checked my code using cargo clippy and reports no warnings
• I have added tests that prove my fix is effective or that my feature works
• I have introduced no new C-bindings
• The changes I've made are Windows, MacOS, UNIX, Linux compatible (or I've handled them using cfg target_os)
• I increased or maintained the code coverage for the project, compared to the previous commit

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@SteveXCIV do the changes address the documentation issue fully, or is there still some problem?

[SteveXCIV]

@hasezoey LGTM - thanks!

[veeso]

@hasezoey Thank you! Please fix conflicts

[hasezoey]

Fixed