regexp: add examples for FindSubmatchIndex and Longest
regexp: add examples for FindSubmatchIndex and Longest

updates #21450

Change-Id: Ibffe0dadc1e1523c55cd5f5b8a69bc1c399a255d
GitHub-Last-Rev: 507f55508121a525de4d210e7ada1396ccaaf367
GitHub-Pull-Request: golang/go#33497
Reviewed-on: https://go-review.googlesource.com/c/go/+/189177
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,akutz/go,golang/go,akutz/go,nawawi/go