Make the Certify screen a little more self-explanatory

Hm, Studio’s indentation on XML seems to disagree with what’s there in some cases.

See bug https://github.com/open-keychain/open-keychain/issues/971
I could live with that, but synchronize suggests 2-way and the reason they’re doing this is to share the certification… doesn’t the word “Share” meet the objective of saying that their certifications are available everywhere?  No biggie