Temporary directory label and description update to remove ambiguity

The temp directory label is "System temp dir"

The temp dir is set via the following vget:

variable_get('radioactivity_temp_dir', sys_get_temp_dir())

Which only pulls the system temp directory if the variable has not been previously set. The title implies that they two temp dirs have more of a connection than they actually do.

Proposed resolution

Update description to match what happens upon variable_get.

User interface changes

Changed field title from "System temporary dir" to "Radioactivity temporary dir". Appended "If no directory is specified, value defaults to system temporary directory." to description.