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

(cherry picked from commit fed6303f29250bd5e656dbdd731b38938c933a61)
Signed-off-by: Matei Zaharia <matei@databricks.com>