Clarify upgrade instructions for Tails 1.1

Originally created by @pilop on #7650 (Redmine)

Users of Tails 1.0.1 actually get an upgrade notification, from the IUK, pointing to this page https://tails.boum.org/doc/first_steps/upgrade/#manual
First, this is not clear enough that translations of this chapter exist, as the links to switch languages only appear at the top of the page.
Moreover, users ask if they should use those instructions or https://tails.boum.org/news/version_1.1/#index5h1

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