doc: clarify type of first argument in zlib

The current documentation for Convenience Methods specifies that
the first argument can be either
`string or buffer`, `string` or `raw Buffer`.
This commit replaces all these instances with `Buffer or string`.

PR-URL: https://github.com/nodejs/node/pull/5685


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: James M Snell <jasnell@gmail.com>