Update man pages for default printer

[zdohnal]

Attempt to make explanation where the default printer is taken from clearer:

$HOME/lpoptions (/etc/cups/lpoptions for root) > ENV variables LPDEST/PRINTER > Server Default

Fixes #819

[zdohnal]

Hopefully fixed by the latest commit - feel free to check.

[michaelrsweet]

I believe we've picked up all of these changes, closing. If there is something I've missed please create a fresh PR or just commit the necessary changes directly. :)