Clarify usage of `Grid::fill` (and derived methods)

[felixhekhorn]

• clarify if the weight argument in Grid::fill relates to $xf(x)$ or just $f(x)$. The subgrid case was discussed in Clarify that imported subgrids have to be provided as convolution with f and not xf #388 - here I face the same problem but for the fill function. I think from here I can deduce that I need $xf(x)$
• consider improving the documentation of the corresponding C function pineappl_grid_fill2 in addition by
  • I would not refer to a deprecated function (pineappl_grid_fill) in the docs for an official one ...
  • the description of the ntuple argument is significantly better on the Rust side "The parameter ntuple must contain the variables specified by the kinematics parameter in the constructor Grid::new in the same order." - just copy that