Simplify and document MatchesPlatform
Simplify and document MatchesPlatform

The function name is still very confusing (especially unclear WRT
argument order) but we are uncertain about the final API, so
let's only tinker with it a little.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image