hw/core/cpu: Update CPUClass::mmu_index docstring

hw/core/cpu: Update CPUClass::mmu_index docstring

Since commits 32a8ea12fab..90b7022e698 (target: "Split out
TARGET_env_mmu_index"), target's memory_rw_debug() callbacks
use the target's TARGET_env_mmu_index(), not the generic
CPUClass::mmu_index() callback. Update the documentation.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250401080938.32278-2-philmd@linaro.org>