all: use automatic RFC linking

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>

ssh/agent/client.go

@@ -93,7 +93,7 @@ type ExtendedAgent interface {
 type ConstraintExtension struct {
 	// ExtensionName consist of a UTF-8 string suffixed by the
 	// implementation domain following the naming scheme defined
-	// in Section 4.2 of [RFC4251], e.g.  "foo@example.com".
+	// in Section 4.2 of RFC 4251, e.g.  "foo@example.com".
 	ExtensionName string
 	// ExtensionDetails contains the actual content of the extended
 	// constraint.