Actually answer "why" on the "journalist public key" page. Also note that signing will at least let us know what keyid was used, so users needing anonymity probably shouldn't.
Update "you can encrypt with our public key" bottom msg.

The only people who should really do that are users already familiar w/PGP (as noted in the "why encrypt with key" page that we link to here). So let's make that explicit right off the bat so we don't scare people. Also mention that submissions become encrypted upon receipt.
be more explicit about when securedrop receive-end encryption happens. (after it's submitted/when it's received, rather than "when it is submitted"). #488
Merge pull request #488 from mtigas/copy-edits-a

Explain "optionally GPG encrypt w/public key" better