Tweaks to Mesos docs

- Mention Apache downloads first
- Shorten some wording

Author: Matei Zaharia <matei@databricks.com>

Closes #806 from mateiz/doc-update and squashes the following commits:

d9345cd [Matei Zaharia] typo
a179f8d [Matei Zaharia] Tweaks to Mesos docs