nir: document restriction on load_smem_amd constantness
This came up while reviewing !29398 ... Possibly this intrinsic should be renamed to load_smem_constant_amd for consistency with load_global_constant. But if we're not going to convey constantness in the intrinsic name, let's at least document the restriction, because NIR's optimizer relies on it.
(I didn't inspect every call site, but it looks like load_smem_amd is just used for descriptor loads so there's no bug to fix.)