Update description of <lattice> in user's guide.

docs/source/usersguide/input.rst

@@ -636,9 +636,10 @@ Each ``<cell>`` element can have the following attributes or sub-elements:
 ---------------------
 
 The ``<lattice>`` can be used to represent repeating structures (e.g. fuel pins
-in an assembly) or other geometry which naturally fits into a two-dimensional
-structured mesh. Each cell within the lattice is filled with a specified
-universe. A ``<lattice>`` accepts the following attributes or sub-elements:
+in an assembly) or other geometry which naturally fits into a two- or
+three-dimensional structured mesh. Each cell within the lattice is filled with a
+specified universe. A ``<lattice>`` accepts the following attributes or
+sub-elements:
 
   :id:
     A unique integer that can be used to identify the surface.
@@ -650,18 +651,19 @@ universe. A ``<lattice>`` accepts the following attributes or sub-elements:
     *Default*: rectangular
 
   :dimension:
-    Two integers representing the number of lattice cells in the x- and y-
-    directions, respectively.
+    Two or three integers representing the number of lattice cells in the x- and
+    y- (and z-) directions, respectively.
 
     *Default*: None
 
   :lower_left:
-    The coordinates of the lower-left corner of the lattice.
+    The coordinates of the lower-left corner of the lattice. If the lattice is
+    two-dimensional, only the x- and y-coordinates are specified.
 
     *Default*: None
 
   :width:
-    The width of the lattice cell in the x- and y- directions.
+    The width of the lattice cell in the x- and y- (and z-) directions.
 
     *Default*: None