largest-series-product: Describe window size 0
largest-series-product: Describe window size 0

Closes https://github.com/exercism/problem-specifications/issues/1910

We established the tests for window size 0 via
https://github.com/exercism/problem-specifications/pull/170

Window size 0 has caused questions in the past, including but not
limited to:

* https://github.com/exercism/problem-specifications/issues/1638
* https://github.com/exercism/go/issues/349

We have comments on the test cases, but it's not necessarily the case
that everyone will see those comments.

Let's try adding the reasoning to the description.

jmluy/x-common,exercism/x-common,jmluy/x-common,exercism/x-common,jmluy/x-common