Clarify meaning of DefaultExperiment.stepSize for Model Exchange

fixes #1939

docs/2_4_common_schema.adoc

@@ -796,6 +796,7 @@ A tool may ignore this information. +
 _[However, it is convenient for a user that <<startTime>>, <<stopTime>>, <<tolerance>> and <<stepSize>> have already a meaningful default value for the model at hand.]_ +
 Furthermore, for Co-Simulation FMUs the <<stepSize>> defines the preferred <<communicationStepSize>>.
 <<startTime>>, <<stopTime>> and <<stepSize>> refer to the unit of the <<independent>> variable.
+For Model Exchange <<stepSize>> can be used as a default sample interval.
 
 [[ModelVariables,`<ModelVariables>`]]
 ==== Model Variables