Updated --isolation help message for slave.

Now also mentions the --modules flag.

Review: https://reviews.apache.org/r/27560
First version of docs/modules.md.

With bits copied from
https://cwiki.apache.org/confluence/display/MESOS/Mesos+Modules+Developer+Guide.

Review: https://reviews.apache.org/r/27453
Update release guide to mention configure.ac.
Update release guide to mention configuration.md.

To update the flags for master, slave and configure.

Review: https://reviews.apache.org/r/27573
Remove newlines from master and slave help messages.

Review: https://reviews.apache.org/r/27588
Updated docs/configuration.md to reflect current state.

Review: https://reviews.apache.org/r/27556
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