Clarify optional fmi3EvaluateDiscreteStates function call

docs/2_3_common_states.adoc

@@ -583,9 +583,10 @@ a|
 |<<fmi3SetClockEM>>
 
 a|
-* If <<providesEvaluateDiscreteStates>> is `true`, then execute state transition function of the model partitions +
+* <<fmi3EvaluateDiscreteStates>> may only be called if <<providesEvaluateDiscreteStates>> is `true`, and the FMU cannot rely on it being called.
+If not called, latexmath:[\mathbf{f}_{\mathit{disc}}] is evaluated implicitly during <<fmi3UpdateDiscreteStates>>.
+If called, it evaluates the state transition function of the model partitions +
 latexmath:[(\mathbf{x}_{d}) := \mathbf{f}_{\mathit{disc}}({}^\bullet\mathbf{x}_{c+d}, \mathbf{u}_{c+d}, \mathbf{p}, {}^\bullet\mathbf{b}, t)]
-* else function is ignored.
 * For all active Clocks latexmath:[\mathbf{k}], the <<clocked-variable,clocked variables>> latexmath:[\mathbf{v}_{\mathbf{k}}] are part of the set of discrete variables latexmath:[\mathbf{v}_d] computed by latexmath:[\mathbf{f}_{\mathit{disc}}].
 |<<fmi3EvaluateDiscreteStates>>
 
@@ -694,8 +695,10 @@ Function <<fmi3GetNumberOfEventIndicators>>::
 Not allowed in Co-Simulation and Scheduled Execution.
 
 Function <<fmi3EvaluateDiscreteStates>>::
-This function is called to trigger the evaluation of latexmath:[\mathbf{f}_{\mathit{disc}}] to compute the current values of discrete states from previous values.
-The FMU signals the support of <<fmi3EvaluateDiscreteStates>> via the capability flag <<providesEvaluateDiscreteStates>>.
+This function may only be called if <<providesEvaluateDiscreteStates>> is `true`.
+It evaluates latexmath:[\mathbf{f}_{\mathit{disc}}], computing the current discrete states.
+Calling this function is optional and the FMU must not rely on it being called.
+If this call is omitted in a super dense time instant, latexmath:[\mathbf{f}_{\mathit{disc}}] is evaluated implicitly by <<fmi3UpdateDiscreteStates>>.
 +
 [[fmi3EvaluateDiscreteStates,`fmi3EvaluateDiscreteStates`]]
 [source, C]