Clarify time in targetValue (#636)

In `targetValue` expressions for the first period after pre-equilibration, I think we want `time` to be the initial time rather than the time at which steady state was reached.

Author: dweindl

doc/v2/documentation_data_format.rst

@@ -297,6 +297,8 @@ phases, following the logic of the event assignments in a single SBML event.
 
    * For subsequent time periods, the *current* values are taken from the
      simulation results at the end of the preceding time period.
+     A special case is simulation time (``time``),
+     which is set to the start time of the current time period.
 
 2. **Assignment of the evaluated ``targetValues`` to their targets**