Add documentation on using _result=

Adds documentation page on how to use the _result argument to specify
the return type for a non-void slang function.

Fixes #69

Author: jhelferty-nv

docs/index.rst

@@ -56,6 +56,7 @@ If you use SlangPy in a research project leading to a publication, please cite t
     :caption: Basics
 
     src/basics/firstfunctions
+    src/basics/returntype
     src/basics/buffers
     src/basics/textures
     src/basics/nested

docs/src/basics/returntype.rst

@@ -0,0 +1,45 @@
+Return Types
+============
+
+Slangpy can generate different types of container to hold the returned results of a Slang function. This is convenient for getting results in a preferred container type, such as a numpy array, texture, or tensor.
+
+Let's start by reusing the example from :ref:`firstfunctions`.
+
+Shader:
+
+.. code-block::
+
+    // example.slang
+
+    // A simple function that adds two numbers together
+    float add(float a, float b)
+    {
+        return a + b;
+    }
+
+In the original :ref:`firstfunctions` python example, we returned the result in a numpy array. Let's return it as a texture instead:
+
+.. code-block:: python
+
+    ## main.py
+
+    # ... initialization here ...
+
+    # Create a couple of buffers with 128x128 random floats
+    a = np.random.rand(128, 128).astype(np.float32)
+    b = np.random.rand(128, 128).astype(np.float32)
+
+    # Call our function and ask for a texture back
+    result = module.add(a, b, _result='texture')
+
+    # Print the first 5x5 values
+    print(result.to_numpy()[:5, :5])
+
+    # Display the result using tev
+    spy.tev.show(result, name='add random')
+
+Here we use ``_result`` to specify that we want the result to be a texture.
+
+The ``_result`` can be ``'numpy'``, ``'texture'``, or ``'tensor'``. You can also use ``_result`` to specify the type directly, like ``numpy.ndarray``, ``slangpy.Texture``, or ``slangpy.Tensor``. Or you can reuse an existing variable of one of those types by passing it directly.
+
+You'll see more examples using ``_result`` in the rest of this documentation!