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

[brennonyork]

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.

[SparkQA]

Test build #28595 has started for PR 5022 at commit 42a10a9.

• This patch merges cleanly.

[brennonyork]

/cc @rxin @srowen

[SparkQA]

Test build #28595 has finished for PR 5022 at commit 42a10a9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28595/
Test PASSed.

[srowen]

OK, I think this is probably the best bet for improving those links and getting them into the TOC.

[brennonyork]

Agreed, wasn't exactly sure if this would work as I'm not sure what tool is parsing the md files, but figured it was the best option.