(SERVER-160) add known issue to documentation

Copy the known issue about /tmp mounted as 'noexec' from the PE documentation.
I think it would be good to mention how one would set the property when running from a package, e.g., add it to the end of the definition of the `JAVA_ARGS` variable in `/etc/sysconfig/puppetserver` or `/etc/default/puppetserver`, depending upon your distribution and then restart the service.