spec: various minor clarifications

- Allow for a type parameter as length/capacity to make.
- Be slightly more precise in prose for append.
- Add a couple of links.

Change-Id: Ib97e528bab1ab55d271beeeb53d9bb7a07047b9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/391754


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>