iter: improve documentation with iterator example

In introducing iterators, package iter gives an example of how to
use an iterator in a range-over-func loop, but currently does not
give an example of what an iterator implementation might look like.

This change adds the example of map.Keys() before the usage example.
Additionally, it references to the Go blog for further examples,
as well as the language spec about for-range loops.

Fixes #70986

Change-Id: I7108d341d314d7de146b4c221700736c943a9f5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/638895
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

0 files changed, 0 insertions, 0 deletions