added docstring to estimated_sample_size in expan/core/early_stopping.py
@@ -52,7 +52,8 @@ def group_sequential(x,
y (array_like): sample of a control group
spending_function: name of the alpha spending function, currently
supports: 'obrien_fleming'
- estimated_sample_size: self-explanatory
+ estimated_sample_size: sample size to be achieved towards
+ the end of experiment
alpha: type-I error rate
cap: upper bound of the adapted z-score
