Make it clear that -proposed is less safe than -backports

[Scott Ritchie]

Currently it is too easy to select the proposed repository without realizing that it contains potential regressions (as uploads are untested). Similarly, backports is a bit too scary as it's "unsupported", when in truth it's about as well supported as universe (ie "supported by the community"). The text should be updated to something a bit more helpful.

ProblemType: Bug
Architecture: amd64
Date: Sat Nov 28 00:04:01 2009
DistroRelease: Ubuntu 9.10
NonfreeKernelModules: nvidia
Package: software-properties-gtk 0.75.4
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-15.50-generic
SourcePackage: software-properties
Uname: Linux 2.6.31-15-generic x86_64