[MRG+1] ENH: added max_train_size to TimeSeriesSplit (#8282)

* ENH: added max_train_size to TimeSeriesSplit

* FIX: update doctest

* FIX: correct error in the previous update

* FIX: added doctest fix for cross_validation.rst

* FIX: remove errors

* TST: tests updated and default value changed to None

* TST: improve split tests

* FIX: reduce code length