qemu-doc: Update the deprecation information of -tftp, -bootp, -redir and -smb

The information how to update the deprecated parameters was too scarce,
so that some people did not update to the new syntax yet. Provide some
more information to make sure that it is clear how to update from the
old syntax to the new one.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>