vl: improve/fix documentation related to RTC function

Documentation describing -rtc option updated to better match current
implementation and highlight some important specifics.

Signed-off-by: Artem Pisarenko <artem.k.pisarenko@gmail.com>
Message-Id: <1b245c6c0803d4bf11dcbf9eb32f34af8c2bd0b4.1539846575.git.artem.k.pisarenko@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed