[arch-general] Rename 'unstable' to 'developing'

As we know, many packages saying even firefox or openoffice in unstable repo don't mean "unstable", instead, they are quite stable. The name of repo simply doesn't reflect the fact, so how about a rename? It's harmless and may even bring some good point, at least from a marketing point, it may courage more devs or TUs to follow the upstream developing and put more packages there, in return attract more people who are interested in the package to do more (testing) work, which can eventually make the package better quality. Just my 2 cents.