Adding a docstring for the new function.

RE:natcap#272

src/pygeoprocessing/routing/helper_functions.py

@@ -9,6 +9,30 @@
 def extract_streams_d8(
         flow_accum_raster_path_band, flow_threshold, target_stream_raster_path,
         raster_driver_creation_tuple=DEFAULT_GTIFF_CREATION_TUPLE_OPTIONS):
+    """Extract D8 streams based on a flow accumulation threshold.
+
+    Creates an unsigned byte raster where pixel values of 1 indicate the
+    presence of a stream and pixel values of 0 indicate the absence of a
+    stream. Any flow accumulation pixels greater than ``flow_threshold`` are
+    considered stream pixels. Nodata values found in the input flow
+    accumulation raster propagate through to the target stream raster.
+
+    Args:
+        flow_accum_raster_path_band (tuple): A (path, band) tuple indicating
+            the path to a D8 flow accumulation raster and the band index to
+            use.
+        flow_threshold (number): The flow threshold. Flow accumulation values
+            greater than this threshold are considered stream pixels, values
+            less than this threshold are non-stream pixels.
+        target_stream_raster_path (string): Where the target streams raster
+            should be written.
+        raster_driver_creation_tuple (tuple): A tuple where the first element
+            is the GDAL driver name of the target raster and the second element
+            is an iterable of raster creation options for the selected driver.
+
+    Returns:
+        ``None``
+    """
     flow_accum_nodata = get_raster_info(
         flow_accum_raster_path_band[0])['nodata'][
             flow_accum_raster_path_band[1]-1]