math/big: add a comment for SetMantExp

Change-Id: I9ff5d1767cf70648c2251268e5e815944a7cb371
Reviewed-on: https://go-review.googlesource.com/c/go/+/233737
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/float.go

@@ -322,10 +322,11 @@ func (z *Float) SetMantExp(mant *Float, exp int) *Float {
 		mant.validate()
 	}
 	z.Copy(mant)
-	if z.form != finite {
-		return z
+
+	if z.form == finite {
+		// 0 < |mant| < +Inf
+		z.setExpAndRound(int64(z.exp)+int64(exp), 0)
 	}
-	z.setExpAndRound(int64(z.exp)+int64(exp), 0)
 	return z
 }