nir: Document extra image source

I was scratching my head about this for a few minutes until I found the answer
in spirv_to_nir. Hopefully this saves someone else some head scratching in turn.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23120>

src/compiler/nir/nir_intrinsics.py

@@ -641,6 +641,9 @@ atomic3("atomic_counter_comp_swap")
 # argument with the value to be written, and image atomic operations take
 # either one or two additional scalar arguments with the same meaning as in
 # the ARB_shader_image_load_store specification.
+#
+# The last source of many image intrinsics is the LOD. This source is zero
+# unless e.g. SPV_AMD_shader_image_load_store_lod is supported.
 def image(name, src_comp=[], extra_indices=[], **kwargs):
     intrinsic("image_deref_" + name, src_comp=[-1] + src_comp,
               indices=[IMAGE_DIM, IMAGE_ARRAY, FORMAT, ACCESS] + extra_indices, **kwargs)