Fix documentation of _skipped interface

[mpip]

• "set skip_trafos[t]=1 if the t-th serial transformation should be computed, otherwise set skip_trafos[t]=0" is the wrong way around
• for 3d-FFT with 3d decomposition skip_trafos is of length rnk_pm (manual tells that it is always of length rnk_pm+1)