Defer to ICU's knowledge of language-specific grammatical quantity rules.

Also improve the documentation to make it a little less unclear what this
is all about. In particular, explain why the original submitter's complaint
about "zero" never being used in English, is expected behavior.

Bug: 2663392
Change-Id: Iade3b4f5c549ce01a95fd0e7e5c6ea394178eda3