Document start/stop measurement feature

[bburan]

Description:

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#3818

Checklist:

• [ ] Branch: next is for changes and new documentation that will go public with the next ESPHome release. Fixes, changes and adjustments for the current release should be created against current.
• [ ] Link added in /index.rst when creating new documents for new components or cookbook.

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[github-actions[bot]]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days. Thank you for your contributions.

[ssieb]

This needs to be going into next, not current.