#385 improve documentation on chsnks and chunk size parameter

[earthgecko]

• Updated docs with with more info on chunksize
• Updated chunksize docstrings with more info on chunksize

Modified:
docs/text/parallelization.rst
tsfresh/convenience/relevant_extraction.py
tsfresh/feature_extraction/extraction.py
tsfresh/feature_selection/relevance.py
tsfresh/feature_selection/selection.py
tsfresh/transformers/feature_augmenter.py
tsfresh/transformers/relevant_feature_augmenter.py

[coveralls]

Coverage remained the same at 97.444% when pulling 8ab8d12 on earthgecko:improve_chunk_size_docs into abb3237 on blue-yonder:master.

[MaxBenChrist]

Good idea! Can you change that description a little bit? I would write

:class:`multiprocessing.Pool` is parallelisation parameter. One data chunk is defined as a singular time series for one id and one kind. The chunksize is the number of chunks that are submitted as one task to one worker process.  
If you set the chunksize to 10, then it means that one worker task corresponds to calculate all features for 10 id/kind time series combinations.  
If it is set it to None, depending on distributor, heuristics are used to find the optimal chunksize.
The chunksize can have an crucial influence on the optimal cluster performance and should be optimised in benchmarks for the problem at hand.

[earthgecko]

@MaxBenChrist modified as requested, all done,

[MaxBenChrist]

Thx!