strings: add examples for Index functions
strings: add examples for Index functions

Change-Id: Ia0f0c8ab4f2f9e96faad6d88775ae19ca7fae53c
Reviewed-on: https://go-review.googlesource.com/53790
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Avelino <8efd86fb78a56a5145ed7739dcb00c78581c5375@avelino.xxx>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

codestation/go,codestation/go,akutz/go,josharian/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,Cofyc/go,CAFxX/go,Cofyc/go,polyverse-security/go,golang/go,polyverse-security/go,CAFxX/go,christopher-henderson/Go,golang/go,akutz/go,polyverse-security/go,nawawi/go,nawawi/go,josharian/go,polyverse-security/go,Cofyc/go,josharian/go,polyverse-security/go,CAFxX/go,nawawi/go,codestation/go,akutz/go,nawawi/go,Cofyc/go,akutz/go,golang/go,codestation/go,cloudflare/go,codestation/go,golang/go,polyverse-security/go,cloudflare/go,nawawi/go,cloudflare/go,polyverse-security/go,cloudflare/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,golang/go,codestation/go,nawawi/go,akutz/go,Cofyc/go,Cofyc/go,CAFxX/go,akutz/go,josharian/go,CAFxX/go,nawawi/go,polyverse-security/go,akutz/go,codestation/go,cloudflare/go,josharian/go,golang/go,golang/go,akutz/go,josharian/go,codestation/go,Cofyc/go,CAFxX/go,cloudflare/go,Cofyc/go