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_unix.go

@@ -149,16 +149,19 @@ func Write(fd int, p []byte) (n int, err error) {
 // creation of IPv6 sockets to return EAFNOSUPPORT.
 var SocketDisableIPv6 bool
 
+// Sockaddr represents a socket address.
 type Sockaddr interface {
 	sockaddr() (ptr unsafe.Pointer, len _Socklen, err error) // lowercase; only we can define Sockaddrs
 }
 
+// SockaddrInet4 implements the Sockaddr interface for AF_INET type sockets.
 type SockaddrInet4 struct {
 	Port int
 	Addr [4]byte
 	raw  RawSockaddrInet4
 }
 
+// SockaddrInet6 implements the Sockaddr interface for AF_INET6 type sockets.
 type SockaddrInet6 struct {
 	Port   int
 	ZoneId uint32
@@ -166,6 +169,7 @@ type SockaddrInet6 struct {
 	raw    RawSockaddrInet6
 }
 
+// SockaddrUnix implements the Sockaddr interface for AF_UNIX type sockets.
 type SockaddrUnix struct {
 	Name string
 	raw  RawSockaddrUnix