Update "client authorization" documentation with new terminology

Description

There was a recent update documentation to use the new client auth terminology (#1476) · Arti: "restricted discovery".

This should be reflected in the user documentation.

Tasks

• Update the relevant documentation, making sure to explain that restricted discovery is not "real authorization" -- it does not have all the required guarantees for such a system (and that's why makes sense to rename it from "client auth"). It's mainly a best effort prevention for unauthorized access, plus an additional DoS defense. An improved mechanism is discussed at tpo/core/arti#1028. Existing docs are:
  • Tor Project | Client Authorization.
  • What's a client or onion authentication? | Tor Project | Support.
  • ONION SERVICES | Tor Project | Tor Browser Manual.
  • Some places in the Ecosystem docs.
• Handle new and existing URLs. Options:
  • Keep the existing URLs, perhaps making redirections if possible.
  • Leave "client authorization" as an alias, as people might still use it for a while.
  • Need to coordinate with Applications Team, to check if there are any strings to be updated.
• Other:
  • Maybe write a quick forum post about this.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)