document iterator for Matrix classes (since they allow short-circuit unlike `forEach()`)

At the moment, the callback in a `forEach()` invocation is simply called and its return value is ignored. However, some operations one might execute using `forEach()`, especially those of the nature of searching for an entry with a desired property, could benefit from the ability to terminate the iteration over the elements early.

Therefore, I would suggest having some sentinel value that could be returned from a forEach callback that would mean the iteration should terminate. Some possibilities:

* Any truthy value. Note this might break existing code, if there are callbacks out there that don't want to terminate, but happen to return a truthy value.
* A special Symbol generated for the purpose. This would not break existing code.
* A plain object `{terminate: true}`. This is _unlikely_ to break existing code, and allows the possibility of specifying a return value as well. (The more-complicated version of this option would use a special Symbol in place of `true` as the value of the `terminate` key to make it impossible to break existing code.)

In addition, we could allow the forEach function to return a value based on the return value of the callback. Some possibilities:
* The return value of the callback the last time it is called is returned from forEach.
* The last time the callback returns a plain object with key `return`, the value of that key is returned.

If the last option in each of the bullets above were adopted, then a callback could return `{terminate: true, return: 0}` to stop the iteration and return 0 from the `forEach` call (for example). Note choosing the first option from each bullet point would allow `forEach` to terminate and return a truthy value, but there would be no way to terminate and return a falsy value.