doc/go1.11: update runtime/trace to use region instead of span
doc/go1.11: update runtime/trace to use region instead of span

The final API uses 'region' instead of 'span' from the proposal.

Change-Id: I305da891a360596fff89b10bc6de3090289b5396
Reviewed-on: https://go-review.googlesource.com/124815
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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