Add comment for npm env vars

Author: NickColley

bin/update-changelog.mjs

@@ -1,5 +1,7 @@
 import { updateChangelog } from '../shared/github-scripts/changelog-release-helper.mjs'
 
+// This file is called automatically after `npm version` runs and which exposes these
+// environment variables (https://github.com/npm/cli/blob/c97b39b1e3436cd20a67ab5f4012a5f395c538b9/workspaces/libnpmversion/lib/version.js#L100-L103)
 const { npm_old_version: previousVersion, npm_new_version: newVersion } =
   process.env