Clarify "intermediary Tails" on /upgrade/tails/

the words “intermediary Tails” can be confusing for some users. usually, this expression is used to describe a Tails device made with dd or UUI, therefore, not a fully featured Tails device.

Related issues

Related to #8589 (closed)
Related to #10614 (closed)
Blocks #15392 (closed)

Original created by @goupille on 12717 (Redmine)

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