Clarify that setting train_size will not change the default value of test_size (0.1), unless you explicitly set it to None.

Only if test_size is explicitly set to None, the value is set to the complement of train_size, not otherwise. This should be made more explicit in the documentation.

@GaelVaroquaux suggested that we could deprecate one of them (preferably train_size?). I feel thats a good idea. Any comments?

See issue #5940