private key on server?

I'm not an expert on crypto. This line in `keybase gen --push` made me nervous:
`Push your encrypted private key to the server? [Y/n] n`
Why would anyone put their private key on the server? (Convenience/backup safety, I'm guessing)
What is it encrypted with?

I really think this should be prefaced with some introduction in-line. At a minimum this should be documented on the site somewhere. I went through the setup/install docs and didn't find anything. The help was far too minimal for `keybase keygen` also.

Review the setup/install documentation and the `keybase gen --push` and `keybase keygen` help text first. Document what happens when an encrypted private key is pushed, what protects it, and add the requested explanation inline or on the site.

Written by the indexing model from the issue text.