The current documentation of chardev mux=on is rather brief and opaque;
expand it to hopefully be a bit more helpful.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1455643738-6068-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>