tcg: Fix documentation for tcg_constant_* vs tcg_temp_free_*

tcg: Fix documentation for tcg_constant_* vs tcg_temp_free_*

At some point during the development of tcg_constant_*, I changed
my mind about whether such temps should be able to be passed to
tcg_temp_free_*.  The final version committed allows this, but the
commentary was not updated to match.

Fixes: c0522136adf
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>