  We derive universal relations for the radio-frequency (rf) spectroscopy of a
two-dimensional Fermi gas consisting of two spin states with a resonant S-wave
interaction. The rf transition rate has a high-frequency tail that is
proportional to the contact and displays logarithmic scaling violations,
decreasing asymptotically like $1/(\omega^2 \ln^2 \omega)$. Its coefficient is
proportional to $\ln^2(a_{2D}'/a_{2D})$, where $a_{2D}$ and $a_{2D}'$ are the
2-dimensional scattering lengths associated with initial-state and final-state
interactions. The clock shift is proportional to the contact and to
$\ln(a_{2D}'/a_{2D})$. If $|\ln(a_{2D}'/a_{2D})| \gg 1$, the clock shift arises
as a cancellation between much larger contributions proportional to
$\ln^2(a_{2D}'/a_{2D})$ from bound-bound and bound-free rf transitions.
