chains: added FIXME note to use autocorrelation for effective_sample_size
chains: added FIXME note to use autocorrelation for effective_sample_size

stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan