bug#45911: authorized-fields is not/badly documented

[raingloom <raingloom@riseup.net>]

guix archive --authorize started issuing a warning some time ago
pointing to "authorized-keys" in "operating-system".

* that is not a valid field of operating-system
* there is no such item in the Guix info page's index
* the relevant example loads a single key from a file, without
  indicating what the syntax of the file is
* trying to store /etc/guix/signing-key.pub as a Scheme file results in
  a parser error due to the hexadecimal syntax being incompatible with
  some Scheme syntax weirdness

So, how the hecc do I add keys permanently the Official Way? Because I
have no idea. I'll try to update the docs when I figure it out.
OOoor... the person who introduced the change could document it. :|