doc/go1.13: describe new 'go version' behavior
doc/go1.13: describe new 'go version' behavior

Change-Id: I18e2546f89e68e77d6e829acc997851751a44f0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/183983
Run-TryBot: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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