all: gofmt

Gofmt to update doc comments to the new formatting.

For golang/go#51082.

Change-Id: I554ad01aa12e84301e44e15c3dc04f82c7244efb
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/399609
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>