math/big: document rounding for Rat.FloatToString

Fixes #11523.

Change-Id: I172f6facd555a1c6db76f25d5097343c20dea59a
Reviewed-on: https://go-review.googlesource.com/12507Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/math/big/ratconv.go

@@ -205,7 +205,8 @@ func (x *Rat) RatString() string {
 }
 
 // FloatString returns a string representation of x in decimal form with prec
-// digits of precision after the decimal point and the last digit rounded.
+// digits of precision after the decimal point. The last digit is rounded to
+// nearest, with halves rounded away from zero.
 func (x *Rat) FloatString(prec int) string {
 	if x.IsInt() {
 		s := x.a.String()

src/math/big/ratconv_test.go

@@ -113,6 +113,8 @@ var floatStringTests = []struct {
 	{"1", 0, "1"},
 	{"1", 2, "1.00"},
 	{"-1", 0, "-1"},
+	{"0.05", 1, "0.1"},
+	{"-0.05", 1, "-0.1"},
 	{".25", 2, "0.25"},
 	{".25", 1, "0.3"},
 	{".25", 3, "0.250"},