Default setting for tmp dir doesn't work behind load balancer

On a setup with multiple servers behind a load balancer, the default tmp directory setting will cause files not to get uploaded under certain circumstances. The solution is to create a tmp directory under the files directory and then use an absolute path to that directory on the settings page. I suggest we update the description text below that field to mention that, or better, have the file system create and use that directory by default rather than /tmp. Granted, not a common problem, but it affects things in a weird way and won't be easily discovered by most people (took me a couple of hours to figure it out, since it manifests itself just as random failed uploads with no errors or anything).