Fleshing out linear decay kernel docstring.

RE:natcap#268

src/pygeoprocessing/kernels.py

@@ -108,8 +108,30 @@ def _exponential_decay(dist):
     )
 
 
-def linear_decay_kernel(target_kernel_path, max_distance, pixel_radius=None,
-                        normalize=True):
+def linear_decay_kernel(
+        target_kernel_path: Text,
+        max_distance: Union[int, float],
+        pixel_radius: Union[int, float] = None,
+        normalize: bool = True) -> None:
+    """Create a linear decay kernel.
+
+    Args:
+        target_kernel_path: The path to where the kernel will be written.
+            Must have a file extension of ``.tif``.
+        max_distance: The maximum distance of the kernel, in pixels. Kernel
+            pixels that are greater than ``max_distance`` from the centerpoint
+            of the kernel will have values of ``0.0``.
+        pixel_radius: The radius of the target kernel, in pixels.  If ``None``,
+            then ``math.ceil(max_distance)`` will be used.
+        normalize: Whether to normalize the kernel.
+
+    Returns:
+        ``None``
+    """
+
+    def _linear_decay(dist):
+        return (max_distance - dist) / max_distance
+
     create_kernel(
         target_kernel_path=target_kernel_path,
         function="(max_dist - dist) / max_dist",