Recommend using the nickname as fallback value for USER params

[emersion]

This documents a widely implemented practice. This allows clients to discover when this field is set to a meaningful value, and when it's not.

[progval]

Seems reasonable.

This may lead developers to (falsely) believe that a majority of existing clients follow this practice. So I wonder if we should add a warning like "But other clients should not assume a realname != nick means the real name was manually set". This would only be useful to deter developers from hiding the nick entirely; but I doubt anyone would do it anyway.

[jwheare]

What if a client has access to the user's actual real name via the OS? Seems that would be a more sensible default. I'm not sure I agree with this SHOULD. It makes sense as a default fallback though in favour of other things like "Android User" or whatever.

[emersion]

I've reworded this to avoid using the word "default".

[jwheare]

LGTM