add note on grain size dependence for xfit_premelt to docs

[chrishavlin]

https://vbr-calc.github.io/vbr/vbrmethods/visc/xfitpremelt/ should describe the grain size dependence. The comment in Params_Viscous.m is kinda buried:

    params.m=3; % grain size exponent
    params.dg_um_r=.004*1e6; % reference grain size [um]
    % note that in YT2016 (section 4.4), YT2016 fit for H, V following
    % Priestly & McKenzie EPSL 2013 and dg_um = dg_um_r. This assumes that
    % the grain size is at the mean grain size of the upper mantle, which
    % Priestly & McKenzie calculate as 4 mm.