When running nixos-rebuild -I, the man page just said "-I path" which could be interpreted as
"just a path to nixpkgs", which in fact it actually has the same meaning as NIX_PATH.
This is now made clear in the manual, so that when grepping "-I" and "NIX_PATH" one quickly finds
the format of the option.
I don't know how to link to the "nix manual" as stated in the docbook, so I left that as it is.
Additionally, it wasn't clear to me how to actually build the man pages and view the changes I made.
That's now in the contributing-to-this-manual.chapter.md.
