doc: sort map output in Effective Go

doc: sort map output in Effective Go

And explain that it does this. A minor change probably worth mentioning,
although (#28782) I'd still like to freeze this document against any substantial
changes.

Fix #30568.

Change-Id: I74c56744871cfaf00dc52a9b480ca61d3ed19a6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/165597
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>