Improve the Resource.getQuantityString/getQuantityText documentation.

Natural languages differ so much and in such odd ways that you can't
use getQuantityString as an "if" statement. It's really just for grammaticality.
This is explained well in
http://developer.android.com/guide/topics/resources/string-resource.html#Plurals
but we need to make more of an effort to motivate people to read that, and to
at least get the most important point across if they don't read it.

Change-Id: I549b9f3563462c45f2dea34c558185e0714127cd