Improve documentation of conditional_variance_decomposition and moments

1. The option conditional_variance_decomposition in stoch_simul only triggers the decomposition, if theoretical moments are requested, i.e. periods=0. We should add a warning in the code and in the manual
2. Even if one specifies order=2 (with periods=0), Dynare still outputs a variance_decomposition, but the decomposition is based on the first order state-space system. We should add a warning for all second order moments as the same is true for variance, correlations, and autocorrelations