add comment to FPT in all-versions

Author: sarahs

lib/all-versions.js

@@ -7,7 +7,8 @@ const versionDelimiter = '@'
 const latestNonNumberedRelease = 'latest'
 
 const plans = [
-  {
+  { // free-pro-team is **not** a user-facing version and is stripped from URLs.
+    // See lib/remove-fpt-from-path.js for details.
     plan: 'free-pro-team',
     planTitle: 'GitHub.com',
     releases: [latestNonNumberedRelease],