math/big: document that Rat.SetString accepts _decimal_ float representations

Updates #29799.

Change-Id: I267c2c3ba3964e96903954affc248d0c52c4916c
Reviewed-on: https://go-review.googlesource.com/c/158397Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/big/ratconv.go

@@ -38,8 +38,8 @@ func (z *Rat) Scan(s fmt.ScanState, ch rune) error {
 }
 
 // SetString sets z to the value of s and returns z and a boolean indicating
-// success. s can be given as a fraction "a/b" or as a floating-point number
-// optionally followed by an exponent. The entire string (not just a prefix)
+// success. s can be given as a fraction "a/b" or as a decimal floating-point
+// number optionally followed by an exponent. The entire string (not just a prefix)
 // must be valid for success. If the operation failed, the value of z is
 // undefined but the returned value is nil.
 func (z *Rat) SetString(s string) (*Rat, bool) {
@@ -78,6 +78,7 @@ func (z *Rat) SetString(s string) (*Rat, bool) {
 	}
 
 	// mantissa
+	// TODO(gri) allow other bases besides 10 for mantissa and exponent? (issue #29799)
 	var ecorr int
 	z.a.abs, _, ecorr, err = z.a.abs.scan(r, 10, true)
 	if err != nil {