Previously, callPackage would try and fill the arguments such as `name`
and `src` which would cause problems if those existed as top-level
attributes. This also makes it clearer what part is the function
signature.

Then document the derivation inline in the code to explain the ellipsis
and various use-cases.

The only reason to pass build inputs is to extend the unpackPhase with
custom unpack commands. Eg: add "unrar" to unpack rar sources. And those
should really be passed as native build inputs. Why? Because
nativeBuildInputs is for dependencies that are used at build time but
will not propagate as runtime dependencies. And also, cross-compilation.

srcOnly: fix invocation and document