strconv: add comment explaining bounded shift in formatBits

The compiler can generate better code for shifts bounded to be less than 32
and thereby known to be less than any register width.
See https://golang.org/cl/109776.

Change-Id: I0c4c9f0faafa065fce3c10fd328830deb92f9e38
Reviewed-on: https://go-review.googlesource.com/c/111735


Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>