SDK documentation should point to "orNull" variants of iterator methods

[stereotype441]

(Based on discussion at #42021 (comment))

A common usage pattern prior to null safety was to pass () => null as the orElse parameter to one of the Iterator methods firstWhere, lastWhere, or singleWhere. With null safety, this doesn't work if the iterator element type is non-nullable, so we introduced the extension methods firstWhereOrNull, lastWhereOrNull, and singleWhereOrNull to package:collection to ensure that users would still be able to get the behavior they want.

But we didn't add any documentation to firstWhere, lastWhere, or singleWhere explaining what we did. As a result, users are unlikely to discover the extension methods by themselves.

We should update the documentation for firstWhere, lastWhere, and singleWhere with information about the "orNull" variants of these methods.

[lrhn]

Good idea.
We usually try to keep the platform libraries self-contained, but comment references to "core" packages is reasonable.