doc/go1.13: document _ between digits for math/big, strconv

Document that:
* math/big.Float.Parse
* math/big.Int.SetString
* strconv.ParseFloat
* strconv.ParseInt
* strconv.ParseUint
now accept underscores to group digits only if base = 0,
as per the Go 2 language changes.

Updates #32815

Change-Id: Id45bd803a18442436419739297e8aed0d32ca56c
Reviewed-on: https://go-review.googlesource.com/c/go/+/191077


Reviewed-by: Robert Griesemer <gri@golang.org>