Add inline comment

Signed-off-by: Adam Li <adam2392@gmail.com>

sklearn/tree/_classes.py

@@ -1386,6 +1386,9 @@ def partial_fit(self, X, y, sample_weight=None, check_input=True, classes=None):
         monotonic_cst = self.monotonic_cst_
 
         # Build tree
+        # Note: this reconstructs the builder with the same state it had during the initial fit.
+        # This is necessary because the builder is not saved as part of the class, and thus
+        # the state may be lost if pickled/unpickled.
         n_samples = X.shape[0]
         criterion = self.criterion
         if not isinstance(criterion, BaseCriterion):