doc/go1.14: mention new math/bits functions Rem, Rem32, Rem64
Updates #28970
Updates #36878
Change-Id: I9676f50516dd5b32bd4e44be136fcb9f43776edd
Reviewed-on: https://go-review.googlesource.com/c/go/+/217127
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>