Improve documentation for fair scores

[andrew-MET]

"Fair" scores are calculated in ens_verify(), ens_crps() and ens_brier() by setting num_ref_members to a value that isn't NA. This gives the "ensemble adjusted" score, with the CRPS and Brier Score scaled to simulate the number of ensemble members set in num_ref_members. A special case of this is setting num_ref_members = Inf, which gives the so-called "fair" score.

This needs to be explained in the documentation for all functions.