Commit 609ad705 ("tcg: Split trunc_shr_i32 opcode into
extr[lh]_i64_i32") remove trunc_shr_i64_i32(). Update the
backend documentation.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230822162847.71206-1-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>