Disclaim scale variations limit

[alecandido]

Scale variations are only accounting for a single patch and they will never do a more complicate job.

While in NNPDF/eko#129 we're going to implement the expanded single patch as a first approximation, in yadism there is no chance we'll introduce something more complicated.

Since eko expanded scale variations are 100% equivalent to apply factorization scale variations on yadism side, if anything more involved is needed it should be done on eko side (in other words: we're not going to implement twice the same complex feature to have it on both sides).
In any case, PineAPPL and the other interpolation grids format would not support the storage of multiple scale variations logs and their recombination (you will have one coefficient for each log(xif)^j log(xir)^k, not all those corresponding to a complicated path).

We should write this disclaimer explicitly in the docs.