Add missing comment [skip ci]

src/pystencils/codegen/config.py

@@ -381,7 +381,10 @@ class GpuOptions(ConfigBase):
     """
 
     max_block_sizes: BasicOption[tuple[int, int, int] | _AUTO_TYPE] = BasicOption()
-    """Maximum dimension size of a thread block in dimensions (x, y, z)."""
+    """Maximum dimension size of a thread block in dimensions (x, y, z).
+    
+    If set to `None` (the default), the default value for the given target will be automatically used.
+    """
 
     manual_launch_grid: BasicOption[bool] = BasicOption(False)
     """Always require a manually specified launch grid when running this kernel.