Document screen locker

It’s now possible to lock the screen in Tails 2.0 from the keyboard shortcut Super+L. But not yet from the system menu, see #5878 (closed).

We should document this. The first step would be to draft a plan. From the top of my head:

• Where would this go?
• Shall we document the limitation (no UI)?
• Is there any other way to lock the screen than the keyboard shortcut?
• Do we want to mention the security limitations of screen locking?

team: sajolida, spriver

Feature Branch: doc/10970-screen-locker

Parent Task: #5684 (closed)

Related issues

• Blocked by #15369 (closed)

Original created by @sajolida on 10970 (Redmine)