pkgsite automatically links /RFC \d+/ to the mentioned RFC. Insert a
bunch of spaces into doc-comments for that to match.

Change-Id: I01834d7573428563f21c37e43316442e148dd8c4
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/442055
Reviewed-by: Joedian Reid <joedian@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>