Rollup merge of #91617 - nnethercote:improve-List-readability, r=lcnr

Rollup merge of #91617 - nnethercote:improve-List-readability, r=lcnr

Improve the readability of `List<T>`.

This commit does the following.
- Expands on some of the things already mentioned in comments.
- Describes the uniqueness assumption, which is critical but wasn't
  mentioned at all.
- Rewrites `empty()` into a clearer form, as provided by Daniel
  Henry-Mantilla on Zulip.
- Reorders things slightly so that more important things
  are higher up, and incidental things are lower down, which makes
  reading the code easier.

r? ````@lcnr````