style(encode): reword the negative-zero rationale comments

Assisted-by: GLM 5.3

encode.go

@@ -831,7 +831,8 @@ func (e *encoder) writeFloat(v float64) error {
 	case math.IsInf(v, -1):
 		e.buf.WriteString("-inf")
 	case v == 0:
-		// Normalise negative zero to positive zero (TOML has no -0).
+		// Normalise negative zero to positive zero, the contract the output
+		// rules in the documentation state.
 		e.buf.WriteString("0.0")
 	default:
 		s := strconv.FormatFloat(v, 'g', -1, 64)

encode_test.go

@@ -67,7 +67,7 @@ func TestMarshalFloatSpecials(t *testing.T) {
 }
 
 func TestMarshalFloatNormalizesNegativeZero(t *testing.T) {
-	// TOML has no -0; the emitter must normalise negative zero to "0.0".
+	// The output contract normalises negative zero to "0.0".
 	type Cfg struct {
 		Z float64 `toml:"z"`
 	}