Update admin mode and make it more visible

What does this MR do?

Support had a ticket asking about restricting admin access (internal link) and it took me quite some time to track it down in the documentation.

This change

1. Links it off the administrator pages and, just there, makes the analogy with sudo.

2. Documents how to turn off admin mode. This is a little less obvious now it's not in the main UI.

3. States clearly what the timeout is, since it's coded into the product. Related: #332251

4. Modifies instructions to reflect the revised UI design; admin mode is now off the main menu and has been renamed slightly.

   • From 5.5:

     

Related issues

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