Include note regarding masked variable references

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What does this MR do?

Adds a note to the Mask a Variable section to clarify behaviour when a masked variable is referenced by another variable that is unmasked. Raised in response to customer ticket, where confusion was resulting from the limitation on including a $ char in a masked variable, where it seems that it should still be possible to reference variables and mask the value.

Note: Very open to re-wording. This was a challenge to distil into a short note that wasn't more confusing than helpful! Support ticket contains further context if necessary.

Related issues

• https://gitlab.zendesk.com/agent/tickets/535031

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