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>

docs/about/build-platforms.rst

@@ -71,7 +71,10 @@ The project aims to support the most recent major version at all times. Support
 for the previous major version will be dropped 2 years after the new major
 version is released or when the vendor itself drops support, whichever comes
 first. In this context, third-party efforts to extend the lifetime of a distro
-are not considered, even when they are endorsed by the vendor (eg. Debian LTS).
+are not considered, even when they are endorsed by the vendor (eg. Debian LTS);
+the same is true of repositories that contain packages backported from later
+releases (e.g. Debian backports). Within each major release, only the most
+recent minor release is considered.
 
 For the purposes of identifying supported software versions available on Linux,
 the project will look at CentOS, Debian, Fedora, openSUSE, RHEL, SLES and