scripts/update: relax note on parallelism now that nixpkgs-bootstrap updates are atomic

scripts/update

@@ -1,14 +1,10 @@
 #!/usr/bin/env nix-shell
 #!nix-shell -i bash -p findutils -p git -p nix-update
 
-# each update job has to do an entire nix eval, which can be memory intensive; be careful when tuning this
-# N.B.: not all update jobs are actually independent! naive parallelism like this *will* result in
-# failed/partial updates, e.g.:
-# 1. `sane.nixpkgs-bootstrap.master` is being updated: this breaks eval of _all_ other attributes.
-# 2. `sane.otherPackage` is being updated: its updateScript fails to eval `sane.otherPackage.src`,
-#     however this would eval if job 1 and 2 were serialized.
-#
-# TODO: perform parallel updates in some way that the update scripts don't interfere with eachother.
+# each update job has to do an entire nix eval, which can be memory intensive; be careful when tuning this.
+# N.B.: not all update jobs are atomic; parallel updates _may_ cause failed updates if eval of some package `pkgFoo`
+# depends on the `src` field of some library `libBar`, and the two are updated simultaneously.
+# this typically only happens when `libBar` is nix code, and `import`'d.
 PARALLELISM=8
 SELF_PATH=$PWD/$0