DOC TimeSeriesSplit training set size formula precision (scikit-learn#28322)

Co-authored-by: Jérémie du Boisberranger <jeremie@probabl.ai>

Author: ZiadAmerr

sklearn/model_selection/_split.py

@@ -1178,7 +1178,9 @@ class TimeSeriesSplit(_BaseKFold):
     The training set has size ``i * n_samples // (n_splits + 1)
     + n_samples % (n_splits + 1)`` in the ``i`` th split,
     with a test set of size ``n_samples//(n_splits + 1)`` by default,
-    where ``n_samples`` is the number of samples.
+    where ``n_samples`` is the number of samples. Note that this
+    formula is only valid when ``test_size`` and ``max_train_size`` are
+    left to their default values.
     """
 
     def __init__(self, n_splits=5, *, max_train_size=None, test_size=None, gap=0):