`nixpkgs.*System` and `nixpkgs.targetPlatform`

Recently I have seen some post here on the forum where someone seemed to set GCC march and mtune like options through one of the options in the subject.

I wanted to look the details up in the docs today, though I was unable to find detailed information in the NixOS manual, not would the description of the options on NixOS Search help me. There those options are described and documented to accept “string or attribute set”, though the format of the set is not documented, and the string examples look like regular system-tuples.

Can someone point me to a more thorough documentation how the set-version of those options can/has to be used?