docs: Add a note about mixing bootindex with "-boot order"

docs: Add a note about mixing bootindex with "-boot order"

Occasionally the users try to mix the bootindex properties with the
"-boot order" parameter - and this likely does not give the expected
results. So let's add a proper statement that these two concepts
should not be used together.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1488303601-23741-1-git-send-email-thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>