unix: add godoc for Sockaddr* types

Add rudimentary godoc for all existing Sockaddr* types implementing the
Sockaddr interface.

Change-Id: Ida7d1e8756477b54fc773f4bc978002c3d4a376d
Reviewed-on: https://go-review.googlesource.com/89775
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_openbsd.go

@@ -18,6 +18,7 @@ import (
 	"unsafe"
 )
 
+// SockaddrDatalink implements the Sockaddr interface for AF_LINK type sockets.
 type SockaddrDatalink struct {
 	Len    uint8
 	Family uint8