doc: make clear that Go 1.4 is not required for bootstrap

doc: make clear that Go 1.4 is not required for bootstrap

Go 1.4 does not work on some systems, including the most
recent versions of macOS. Make it clearer that that's not the only
way to bootstrap Go.

Change-Id: I7c03d6808e43bf26283a53eab2bb0b2dc4af73af
Reviewed-on: https://go-review.googlesource.com/c/go/+/277216
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>