doc: sort zlib alphabetically

Reorders, with some contextual changes, the zlib documentation
alphabetically.

- Put Convenience Methods children under the Convenience Methods header
- Renamed Options to Class Options and put above the Class definitions

PR-URL: https://github.com/nodejs/node/pull/3662


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>