Mention that it specifically returns x / 2, and do the same for
EncodedLen.

Change-Id: Ie334f5abecbc487caf4965abbcd14442591bef2a
Change-Id: Idfa413faad487e534489428451bf736b009293d6
Reviewed-on: https://go-review.googlesource.com/33191Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>