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
Change-Id: I09eb39ea6986850c3af28cca3052d319e3c97705
Merge "Fix and be more explicit about plurals best practices."