[SPARK-6329][Docs]: Minor doc changes for Mesos and TOC

Updated the configuration docs from the minor items that Reynold had left over from SPARK-1182; specifically I updated the `running-on-mesos` link to point directly to `running-on-mesos#configuration` and upgraded the `yarn`, `mesos`, etc. bullets to `<h5>` tags in hopes that they'll get pushed into the TOC.

Author: Brennon York <brennon.york@capitalone.com>

Closes #5022 from brennonyork/SPARK-6329 and squashes the following commits:

42a10a9 [Brennon York] minor doc fixes

docs/configuration.md

@@ -1391,9 +1391,11 @@ Apart from these, the following properties are also available, and may be useful
 Each cluster manager in Spark has additional configuration options. Configurations
 can be found on the pages for each mode:
 
- * [YARN](running-on-yarn.html#configuration)
- * [Mesos](running-on-mesos.html)
- * [Standalone Mode](spark-standalone.html#cluster-launch-scripts)
+##### [YARN](running-on-yarn.html#configuration)
+
+##### [Mesos](running-on-mesos.html#configuration)
+
+##### [Standalone Mode](spark-standalone.html#cluster-launch-scripts)
 
 # Environment Variables