diff --git a/src/math/big/float.go b/src/math/big/float.go
index b3c3295201..da964eef3e 100644
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -224,7 +224,9 @@ func (x *Float) Mode() RoundingMode {
 	return x.mode
 }
 
-// Acc returns the accuracy of x produced by the most recent operation.
+// Acc returns the accuracy of x produced by the most recent
+// operation, unless explicitly documented otherwise by that
+// operation.
 func (x *Float) Acc() Accuracy {
 	return x.acc
 }
diff --git a/src/math/big/sqrt.go b/src/math/big/sqrt.go
index 53403aa41d..ac2094f28e 100644
--- a/src/math/big/sqrt.go
+++ b/src/math/big/sqrt.go
@@ -14,7 +14,8 @@ var (
 //
 // If z's precision is 0, it is changed to x's precision before the
 // operation. Rounding is performed according to z's precision and
-// rounding mode.
+// rounding mode, but z's accuracy is not computed. Specifically, the
+// result of z.Acc() is undefined.
 //
 // The function panics if z < 0. The value of z is undefined in that
 // case.