Update interface.nix

[aameen-tulip]

Add some commonly used derivation fields.

The only oddball here is outputHash which should not be set on the real derivation if it is unset or null.

If these look like good addition lets me know and I can do the implementation.

[DavHau]

These options already exist under /modules/derivation-common, though without a description fields and examples. Could you just add description/examples there?

Also, should we just export all the documentation from the nix nix manual. Otherwise we have two manuals for the same thing with different content