Update <dagmc_universe> IO format docs for new <cell> sub-element schema

Replace the deprecated <material_overrides> documentation with the new
<cell> sub-element spec covering id, name, material, temperature, density,
and volume. Include unsupported attribute list and a deprecation note for
the old format.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pshriwise

docs/source/io_formats/geometry.rst

@@ -406,24 +406,58 @@ Each ``<dagmc_universe>`` element can have the following attributes or sub-eleme
 
     *Default*: None
 
-  :material_overrides:
-    This element contains information on material overrides to be applied to the
-    DAGMC universe. It has the following attributes and sub-elements:
-
-    :cell:
-      Material override information for a single cell. It contains the following
-      attributes and sub-elements:
-
-      :id:
-        The cell ID in the DAGMC geometry for which the material override will
-        apply.
-
-      :materials:
-        A list of material IDs that will apply to instances of the cell. If the
-        list contains only one ID, it will replace the original material
-        assignment of all instances of the DAGMC cell. If the list contains more
-        than one material, each material ID of the list will be assigned to the
-        various instances of the DAGMC cell.
+  :cell:
+    Zero or more ``<cell>`` sub-elements may appear to override properties of
+    individual DAGMC volumes. Each ``<cell>`` element supports the following
+    attributes and sub-elements:
+
+    :id:
+      The integer cell ID in the DAGMC geometry to override. Required.
+
+    :name:
+      An optional string label for the cell.
+
+      *Default*: None
+
+    :material:
+      The material ID to assign to this cell. Use ``void`` for vacuum. Multiple
+      space-separated IDs may be given to specify a distribmat (distributed
+      material) assignment. May also be specified as a ``<material>``
+      sub-element. Required.
+
+    :temperature:
+      Temperature(s) in Kelvin to assign to the cell. Must be ≥ 0. Multiple
+      space-separated values may be given. May also be specified as a
+      ``<temperature>`` sub-element.
+
+      *Default*: None
+
+    :density:
+      Density in g/cm³ to assign to the cell. Must be > 0. Requires a non-void
+      material fill. Multiple space-separated values may be given. May also be
+      specified as a ``<density>`` sub-element.
+
+      *Default*: None
+
+    :volume:
+      Volume of the cell in cm³.
+
+      .. note:: DAGMC can compute cell volumes exactly from the triangulated
+                mesh surfaces. Specifying a manual volume risks inconsistency
+                with that capability.
+
+      *Default*: None
+
+    The following standard ``<cell>`` attributes are **not** supported inside
+    ``<dagmc_universe>`` and will raise an error if present: ``region``,
+    ``fill``, ``universe``, ``translation``, ``rotation``.
+
+    .. deprecated::
+      The ``<material_overrides>`` sub-element (containing ``<cell_override>``
+      children with ``<material_ids>``) is deprecated. A deprecation warning is
+      emitted and the overrides are converted to the ``<cell>`` format at parse
+      time. It is an error to specify both ``<material_overrides>`` and
+      ``<cell>`` sub-elements on the same ``<dagmc_universe>``.
 
     *Default*: None