docs: add basic iterator example
docs: add basic iterator example

I noticed we had examples for all kinds of advanced iterators, but not
basic iterator usage. This CL adds such an example.

Change-Id: I94a703cc027f59dd01d766c1a4868c1614b622af
Reviewed-on: https://code-review.googlesource.com/c/38011
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>

google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,googleapis/google-api-go-client