Define when-clause clearly

[casella]

Fixes #3840

[HansOlsson]

Good, but not complete.

The when-clause can be traced back to version 1.1 of the specification that in the grammar had when_clause_e (for when-equation) and when_clause_a (for when-algorithm). Thus (non-clocked) when-clause almost certainly is intended to cover both (when applicable).

Section 8.5 even implicitly states this:

All equations and assignment statements within when-clauses and all assignment statements within
function classes are implicitly treated with noEvent, i.e., relations within the scope of these operators never induce state or time events...

[HansOlsson]

It should be clear that when-clauses also include when-statements in some way.

[HansOlsson]

Suggested change

	The \lstinline!when!-equations\index{when@\robustinline{when}!equation}\index{then@\robustinline{then}!when-equation@\robustinline{when}-equation}\index{elsewhen@\robustinline{elsewhen}!when-equation@\robustinline{when}-equation}, also known as \lstinline!when!-clauses, have the following syntax:
	When-clauses\index{when@\robustinline{when}!clause} refer to both \lstinline!when!-equations and \lstinline!when!-statements, \cref{when-statements}.
	The \lstinline!when!-equations\index{when@\robustinline{when}!equation}\index{then@\robustinline{then}!when-equation@\robustinline{when}-equation}\index{elsewhen@\robustinline{elsewhen}!when-equation@\robustinline{when}-equation} have the following syntax:

I know that it should preferably be rewritten, but at least this ensures that both when-equations and when-statements are mentioned.

[HansOlsson]

With rewritten I primarily mean that it should be placed somewhere else (in this section, or possible some other one), but I cannot easily do that as a comment.