Prior to this change, man pages from Nixpkgs written using the mdoc(7)
macros would start like this:
NC(1) BSD General Commands Manual NC(1)
and end like this:
BSD December 27, 2018 BSD
No matter what operating system they were run on.
It's far more accurate to say "Nixpkgs General Commands Manual", so
with this patch we configure groff to do just that. The variable is
called "operating-system", but I think it makes more sense to say
"Nixpkgs" than "NixOS" or something, because packages from Nixpkgs can
run on lots of operating systems, and the important thing is that the
package is from Nixpkgs.
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