Update documentation to use the new client auth terminology

As mentioned in !2212 (merged), we are updating the terminology around client authorization:

• services that encrypt the second layer of their descriptor are said to be running in "restricted discovery" mode
• the corresponding client "auth" keys are referred to as service "discovery keys".

We should update the docs throughout arti (and torspec, where applicable) to reflect this change.

The following discussion from !2212 (merged) should be addressed:

• @nickm started a discussion:

  As a separate ticket, we should probably make sure that the torspec documentation (and anything else in Arti) also calls these keys "service discovery keys". We probably don't want to remove all the old names in the spec for these keys, though, since we don't plan to do a full renaming on C tor and existing APIs.