srcOnly: fix invocation and document

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.

pkgs/build-support/src-only/default.nix

@@ -1,4 +1,19 @@
-{stdenv, name, src, patches ? [], buildInputs ? [], ...}: 
+{ stdenv }@orig:
+# srcOnly is a utility builder that only fetches and unpacks the given `src`,
+# maybe pathings it in the process with the optional `patches` and
+# `buildInputs` attributes.
+#
+# It can be invoked directly, or be used to wrap an existing derivation. Eg:
+#
+# > srcOnly pkgs.hello
+#
+{ name
+, src
+, stdenv ? orig.stdenv
+, patches ? []
+, buildInputs ? []
+, ... # needed when passing an existing derivation
+}:
 stdenv.mkDerivation {
   inherit src buildInputs patches name;
   installPhase = "cp -r . $out";