Make warning about authorizedKeys+NixOps more nuanced

[jleeuwes]

This is a documentation-only change.

Motivation for this change

The warning about authorizedKeys in combination with NixOps is a bit too alarming. I'm using NixOps and provision my own keys, for which using authorizedKeys is just fine. Also, for non-root users there's also no problem.

Things done

Built the manual and visually checked that the change looked okay.

[stale]

I marked this as stale due to inactivity. → More info

[jleeuwes]

What can I do to move this PR forward?

[roberth]

I don't see any obvious evidence that NixOps would even behave this way. I've checked 1.7 and master, but neither seem to use something like lib.mkDefault in their keys definition that would cause this problem.
When the priorities are the same (no mix of mkDefault and regular defs for example) the lists will be combined.

Of course, me reading code doesn't prove much. This needs testing.

[jleeuwes]

@roberth would some manual testing suffice, or are you thinking of an automated test (maybe as part of this PR)? If the latter, could you maybe point me to some documentation about how testing is done in this project?

[roberth]

@jleeuwes sorry for the late response. NixOps testing is split across the nixops repo and its providers; and it could use some improvement. Manual testing is sufficient for this change.