Emphasize the importance of proper event handling

[mikep-dev]

This is a followup to the issue: #502

When batching, attaching a visibilitychange event listener to document can cause CLS data to be missed because CLS data is reported on visibilitychange event on document as well and depending on when event listeners were attached, sometimes data will be reported before or after batching.

This is very easy to miss and some users, including myself, overlooked this (e.g. #500). This PR adds a much needed clarification.

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