ssh-protocols: Document Arti keystore key types

Fixes arti#1109 (closed)

@gabi-250: please would you check that what I have written about the file formats is actually true.

@nickm: please would you check that you agree with what I have written in the non-normative explanations. Also if you can get the { #explicit-anchor-name } thing to work then please demonstrate it to me! Maybe I'm having a brain fart.