compress/{gzip,zlib}: mention that Close flushes

Our other CloseFlushers (archive/tar, compress/flate) do mention this.

The original change was accidentally submitted to the release branch:
        https://codereview.appspot.com/117430043/

TBR=rsc
R=r, rsc
CC=golang-codereviews
https://codereview.appspot.com/124130043