Add QTCHOOSER_NO_GLOBAL_DIR to the man page.

Document what QTCHOOSER_NO_GLOBAL_DIR does in the man page.

Change-Id: I00bb056eba0713ebb70c52a1ad332050ffcca052
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>