net: update IP.MarshalText documentation regarding len(ip) == 0

Describe the difference from String encoding when len(ip) is zero.

Change-Id: Ia9b36b405d4fec3fee9a77498a839b6d90c2ec0d
Reviewed-on: https://go-review.googlesource.com/37379Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/ip.go

@@ -340,7 +340,8 @@ func ipEmptyString(ip IP) string {
 }
 
 // MarshalText implements the encoding.TextMarshaler interface.
-// The encoding is the same as returned by String.
+// The encoding is the same as returned by String, with one exception:
+// When len(ip) is zero, it returns an empty slice.
 func (ip IP) MarshalText() ([]byte, error) {
 	if len(ip) == 0 {
 		return []byte(""), nil