docs: Correct outdated information on QAPI

docs: Correct outdated information on QAPI

* Fix guidance on error classes

* Point to generated documentation

* Drop plea for documentation, because the QAPI code generator
  enforces it since commit 3313b6124b

* Minor tweaks here and there

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180211093607.27351-27-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>