Fix and be more explicit about plurals best practices.

The "one" case should always include %d. Say so, and fix our examples.
Also make it clear that developers should always provide exactly "one"
and "other".

Bug: 18429565

(cherry picked from commit 1fe7d496)

Change-Id: I89656c74b7dfd91584e1bb6d0a3f2da1dda81079