doc: more accurate zlib windowBits information

Fixes: https://github.com/nodejs/node/issues/14847
PR-URL: https://github.com/nodejs/node/pull/16511


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>