sort: update comments

- Describe requirements on Less more precisely.
- Standardize on x for the variable name of the data being sorted
  (was variously a, p, slice).
- Many other minor wording changes.

Fixes #41951.

Change-Id: Ic9e222a53ec035fcc3b5ddfc7f0eefbe1bb2890d
Reviewed-on: https://go-review.googlesource.com/c/go/+/262657


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>