Fixed {print,describe}-object, documentation in src/{protocol,service,provider}.lisp

* src/protocol.lisp (documentation symbol eql service): new method;
  when the symbol names a service, return its documentation
* src/service.lisp (describe-object standard-service t): new method;
  print the service object and its providers
  (documentation standard-service eql t): fixed doc-type specializer t
  -> eql t; just return the documentation instead of making up an
  elaborate documentation string on the fly
* src/provider.lisp (documentation class-provider eql t): new method;
  return documentation of the associated class
  (documentation function-provider eql t): similar