encoding/binary: add examples for varint functions
encoding/binary: add examples for varint functions

Change-Id: I191f6e46b452fadde9f641140445d843b0c7d534
Reviewed-on: https://go-review.googlesource.com/48604
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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