The new math/bits package has a section for itself, and should not be
mentioned in the 'Minor changes to the library' section of the release
notes.

Updates #20587

Change-Id: I13ecd35f5cee4324e50b2d31800e399c00159126
Reviewed-on: https://go-review.googlesource.com/45051Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>