[Docs] Add a note about the units used for the time-based HRV metrics.

[aweinstein]

Description

This PR aims at improving the docstring of hrv_time function. This PR should fix #1046.

Proposed Changes

I added a note to the docstring stating that "Where applicable, the unit used for these metrics is the millisecond."

Checklist

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• [X ] I have read the CONTRIBUTING file.
• [X ] My PR is targeted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.

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