more explanation in composer guide

docs/source/usage/how_to_composer.md

@@ -379,9 +379,16 @@ with SourceFileGenerator(sfg_config) as sfg:
     sfg.function("kernel_wrapper")(
         sfg.gpu_invoke(khandle)
     )
-
 ```
 
+In this snippet, we used the [generator configuration](#how_to_generator_scripts_config)
+to change the suffix of the generated implementation file to `.cu`.
+
+When investigating the generated `.cu` file, you can see that the GPU launch configuration parameters
+*grid size* and *block size* are being computed automatically from the array sizes.
+This behavior can be changed by modifying options in the {any}`gpu <pystencils.codegen.config.GpuOptions>`
+category of the `CreateKernelConfig`.
+
 #### Mapping Fields to Data Structures
 
 Pystencils kernels operate on n-dimensional contiguous or strided arrays,