docs: improved comments for generate-variants.js

I asked claude about this file many many days ago. It wrote some comments and conjured some tests (albeit not using jest)

Author: iAmMichaelConnor

noir-projects/noir-protocol-circuits/Nargo.template.toml

@@ -1,3 +1,7 @@
+# This is a template file.
+# The actual Nargo.toml for this workspace is generated by `./scripts/generate_variants.js` to include
+# the many autogenerated variants of the reset circuit.
+
 [workspace]
 members = [
     "crates/ts-types",