Update documentation related to default options for "known hosts" and "private key"

The documentation is wrong regarding what happens when the "known hosts" and "private key" options are NULL. According to the documentation at http://api.libssh.org/stable/group__libssh__session.html#ga7a801b85800baa3f4e16f5b47db0a73d they both default to the system default, which is not always ~/.ssh/*.

P.S.: I don't know why GitHub keeps telling me that I would like to pull request more than one commit when it is actually only one. I pulled from upstream (rtrlib/rtrlib), pushed to origin (my fork), made my changes, pushed to origin and filed the pull request.