Clarify that VSA only includes items that have reached the end event

What does this MR do?

Many users are confused about how Value Stream Analytics measures the stages. It has been particularly confusing that it only displays items that have reached the end event for the given stage. This adds some clarification to the documentation. We plan to link to this clarified documentation in the app per #326920 (comment 545566164)

Does this MR meet the acceptance criteria?

Conformity

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Security

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Resolves #326920 (closed)