Clarify `Expr.forEachWhere`

[Kha]

As noted in #2491, the function docstring does not sufficiently explain its difference to forEach and in what circumstances one should use it. The module docstring gives more details, but is still misleading: it does not even mention the, I believe, main property of the function, f being applied at most once to each (structurally equal) subexpression. Thus a much more relevant question as to whether the function should be used might be whether f is much more expensive than p plus expr hashing.

Thus I propose to first confirm and document the intent of the function as well as consider renaming it (such as to forEachOnceWhere) before considering any refactors on top of it.