cmd/go: clarify that tag lists are space-separated
Apparently the current documentation is confusing users that
quickly skim the flags list at the top. Make very clear that
build tags are space-separated.
Updates #18800
Change-Id: I473552c5a2b70ca03d8bbbd2c76805f7f82b49a2
Reviewed-on: https://go-review.googlesource.com/35951 Reviewed-by: Daniel Martà <mvdan@mvdan.cc> Reviewed-by: Minux Ma <minux@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>