Document how UnitDelay currently works.

[HansOlsson]

The current description of UnitDelay has two issues:

• It is plain wrong regarding "second"
• It doesn't document the strong restriction that UnitDelay only works for sampled inputs, this is also the issue in Modelica.Blocks.Discrete.UnitDelay does not delay input #4451

An alternative is to just correct the model as described in #4451; the downside is that it is not backwards compatible and it introduces additional states.