Add model attributes objective states to docs

docs/reference/models.rst

@@ -19,6 +19,15 @@ Model Class
 
 .. autoclass:: Model
 
+Model Attributes 
+----------------
+
+.. autosummary::
+    :toctree: generated/
+
+    ~Model.objective
+    ~Model.states
+
 Model Primitives  
 ----------------
 

dwave/optimization/model.pxd

@@ -40,8 +40,38 @@ cdef class Model:
     cdef cppGraph _graph
 
     cdef readonly object objective  # todo: cdef ArraySymbol?
+    """Objective to be minimized.
+
+    Examples:
+        This example prints the value of the objective of a model representing 
+        the simple polynomial, :math:`y = i^2 - 4i`, for a state with value 
+        :math:`i=2.0`. 
+
+        >>> from dwave.optimization import Model
+        ...
+        >>> model = Model()
+        >>> i = model.integer(lower_bound=-5, upper_bound=5)
+        >>> c = model.constant(4)
+        >>> y = i**2 - c*i
+        >>> model.minimize(y) 
+        >>> with model.lock():
+        ...     model.states.resize(1)
+        ...     i.set_state(0, 2.0)
+        ...     print(f"Objective = {model.objective.state(0)}")
+        Objective = -4.0
+    """
 
     cdef readonly States states
+    """States of the model.
+
+    :ref:`States <intro_optimization_states>` represent assignments of values
+    to a symbol.
+
+    See also:
+        :ref:`States methods <optimization_models>` such as 
+        :meth:`~dwave.optimization.model.States.size` and 
+        :meth:`~dwave.optimization.model.States.resize`.
+    """
 
     # The number of times "lock()" has been called.
     cdef readonly Py_ssize_t _lock_count