docs: Improvement to prior distributions section

[haakon-e]

It would be useful if, in prior distributions section, it was explicitly made clear that you need to first type

using EnsembleKalmanProcesses.ParameterDistributions

to be able to call e.g. constrained_gaussian. Currently, this is only implicitly implied in the intro, e.g.:

[...] This page describes the interface EnsembleKalmanProcesses provides for specifying priors on parameters, via the ParameterDistributions module (src/ParameterDistributions.jl).