assert: add comments for diff algorithm

It became hard to follow what was actually happening in the
algorithm. This adds comments to improve the situation.

PR-URL: https://github.com/nodejs/node/pull/23048
Refs: https://github.com/nodejs/node/issues/22763


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>