strconv: ParseFloat, ParseInt, ParseUint docs don't show that they accept underscores to group digits

[odeke-em]

We added support for underscores to group digits as per Go 2 language changes for strconv in 36a81d5#diff-15cb26824bae5fecc60070ada211e995 aka CL https://go-review.googlesource.com/c/160243
but unfortunately we didn't update the docs
as per

I noticed this while working on CL https://go-review.googlesource.com/c/go/+/191077

[gopherbot]

Change https://golang.org/cl/191078 mentions this issue: strconv: document _ grouping in digits for Parse(Float, *Int)

[gopherbot]

Change https://golang.org/cl/191168 mentions this issue: [release-branch.go1.13] strconv: update documentation