trivial: make symlinkJoin support pname+version alone

doc/build-helpers/trivial-build-helpers.chapter.md

···
 
 ## `symlinkJoin` {#trivial-builder-symlinkJoin}
 
-This can be used to put many derivations into the same directory structure. It works by creating a new derivation and adding symlinks to each of the paths listed. It expects two arguments, `name`, and `paths`. `name` is the name used in the Nix store path for the created derivation. `paths` is a list of paths that will be symlinked. These paths can be to Nix store derivations or any other subdirectory contained within.
+This can be used to put many derivations into the same directory structure. It works by creating a new derivation and adding symlinks to each of the paths listed. It expects two arguments, `name`, and `paths`. `name` (or alternatively `pname` and `version`) is the name used in the Nix store path for the created derivation. `paths` is a list of paths that will be symlinked. These paths can be to Nix store derivations or any other subdirectory contained within.
 Here is an example:
 ```nix
 # adds symlinks of hello and stack to current build and prints "links added"

pkgs/build-support/trivial-builders/default.nix

···
     as a easy way to build multiple derivations at once.
    */
   symlinkJoin =
-    args_@{ name
+    args_@{
+      name ?
+        assert lib.assertMsg (args_ ? pname && args_ ? version)
+          "symlinkJoin requires either a `name` OR `pname` and `version`";
+        "${args_.pname}-${args_.version}"
     , paths
     , preferLocalBuild ? true
     , allowSubstitutes ? false