Cleanup: Remove outdated comment in request.rb about instance-memoization

Spotted an outdated doc-comment that was otherwise a bit confusing.

* Removed memoization in: 0bf8d8e840ae226d6efed8af345feb67a10f6aa6
* Added memoization + comment in: eefbed89c4ece749e889132012d0f67cd87926a8