Docstring for ```sampler.is_exact``` is not... exact

Created by: gcarleo

This docstring is a bit ambiguous: it says

The sampler is exact if the samples are always distributed according to the chosen power of the variational state.

However I would argue that the sampler is exact if all the samples are exactly distributed according to the chosen power of the variational state and (also, crucially) there is no correlation among them