Clarify what is Tails Greeter in the documentation of each additional setting

During the UX design of Additional Software, the two people who read https://tails.boum.org/doc/first_steps/startup_options/administration_password/ but were not very familiar with Tails didn’t understand what Tails Greeter was (nor did they click on the Tails Greeter link).

Original created by @sajolida on 15260 (Redmine)

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