strconv: adds missing comment to neg flag, formats comment on trunc flag

Change-Id: Ibdd57489543d57a24d1e3c41210abd9fbc930b8f
Reviewed-on: https://go-review.googlesource.com/48867Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/strconv/decimal.go

@@ -15,7 +15,7 @@ type decimal struct {
 	d     [800]byte // digits, big-endian representation
 	nd    int       // number of digits used
 	dp    int       // decimal point
-	neg   bool
+	neg   bool      // negative flag
 	trunc bool      // discarded nonzero digits beyond d[:nd]
 }