Clarify corner case behavior of --device option

doc/src/vpr/command_line_usage.rst

@@ -200,12 +200,14 @@ General Options
 
 .. option:: --device <string>
 
-    Specifies which device layout/floorplan to use from the architecture file.
+    Specifies which device layout/floorplan to use from the architecture file.  Valid values are:
 
-    ``auto`` uses the smallest device satisfying the circuit's resource requirements.
-    Other values are assumed to be the names of device layouts defined in the :ref:`arch_grid_layout` section of the architecture file.
+    * ``auto`` VPR uses the smallest device satisfying the circuit's resource requirements.  This option will use the ``<auto_layout>`` tag if it is present in the architecture file in order to construct the smallest FPGA that has sufficient resources to fit the design. If the ``<auto_layout>`` tag is not present, the ``auto`` option chooses the smallest device amongst all the architecture file's ``<fixed_layout>`` specifications into which the design can be packed.
+    * Any string matching ``name`` attribute of a device layout defined with a ``<fixed_layout>`` tag in the :ref:`arch_grid_layout` section of the architecture file.
 
-    .. note:: If the architecture contains both ``<auto_layout>`` and ``<fixed_layout>`` specifications, specifying an ``auto`` device will use the ``<auto_layout>``.
+    If the value specified is neither ``auto`` nor matches the ``name`` attribute value of a ``<fixed_layout>`` tag, VPR issues an error.
+
+    .. note:: If the only layout in the architecture file is a single device specified using ``<fixed_layout>``, it is recommended to always specify the ``--device`` option; this prevents the value ``--device auto`` from interfering with operations supported only for ``<fixed_layout>`` grids.
 
     **Default:** ``auto``