Clarify native snippet examples

Review bots noted that two native snippet examples were less clear
than they should be. The differentiability example used positional
decorator arguments even though the surrounding prose names
adj_snippet, and the shared-memory example launched with arr and
out without defining them in the same block.

Use keyword arguments for adj_snippet examples and make the
shared-memory launch snippet define its inputs before launch.
This keeps the docs easier to copy and less dependent on decorator
argument order.

Signed-off-by: Eric Shi <ershi@nvidia.com>

Author: shi-eric

docs/user_guide/differentiability.rst

@@ -672,7 +672,7 @@ an adjoint implementation for the native operation. Provide it with the
     adj_y[tid] += adj_out[tid];
     """
 
-    @wp.func_native(snippet, adj_snippet)
+    @wp.func_native(snippet=snippet, adj_snippet=adj_snippet)
     def axpy(
         x: wp.array[float],
         y: wp.array[float],

docs/user_guide/extending_warp.rst

@@ -63,6 +63,9 @@ one result per block.
 
 .. code-block:: python
 
+    import numpy as np
+    import warp as wp
+
     snippet = """
         __shared__ int sum[128];
 
@@ -93,6 +96,8 @@ one result per block.
         reduce(arr, out, tid)
 
 
+    arr = wp.array(np.arange(128, dtype=np.int32), dtype=wp.int32, device="cuda")
+    out = wp.zeros(1, dtype=wp.int32, device="cuda")
     wp.launch(reduce_kernel, dim=128, inputs=[arr], outputs=[out], block_dim=128, device="cuda")
 
 Returning Values
@@ -131,7 +136,7 @@ variables use the ``adj_`` prefix, and return-value adjoints are named
         """
 
 
-    @wp.func_native(snippet, adj_snippet)
+    @wp.func_native(snippet=snippet, adj_snippet=adj_snippet)
     def axpy(
         x: wp.array[wp.float32],
         y: wp.array[wp.float32],

warp/_src/context.py

@@ -1080,7 +1080,7 @@ def reverse_block(arr: wp.array[int], out: wp.array[int], tid: int): ...
                 '''
 
 
-            @wp.func_native(snippet, adj_snippet)
+            @wp.func_native(snippet=snippet, adj_snippet=adj_snippet)
             def axpy(
                 x: wp.array[wp.float32],
                 y: wp.array[wp.float32],