docs: build-platforms: Clarify stance on minor releases and backports

docs: build-platforms: Clarify stance on minor releases and backports

These changes match those made in the following libvirt commits:

  2ac78307af docs: Clarify our stance on backported packages
  78cffd450a docs: Spell out our policy concerning minor releases

Since QEMU's platform support policy is based on libvirt's, it
makes sense to mirror these recent changes made to the latter.

The policy is not altered significantly - we're simply spelling
out some rules that were likely already being implicitly
enforced.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>