Update the docs ops.py (microsoft#718)

* Update ops.py

* Update ops.py

qlib/data/ops.py

@@ -689,6 +689,9 @@ def get_extended_window_size(self):
 
 class Rolling(ExpressionOps):
     """Rolling Operator
+    The meaning of rolling and expanding is the same in pandas.
+    When the window is set to 0, the behaviour of the operator should follow `expanding`
+    Otherwise, it follows `rolling`
 
     Parameters
     ----------
@@ -1194,6 +1197,14 @@ def _load_internal(self, instrument, start_index, end_index, freq):
 # support pair-wise rolling like `Slope(A, B, N)`
 class Slope(Rolling):
     """Rolling Slope
+    This operator calculate the slope between `idx` and `feature`.
+    (e.g. [<feature_t1>, <feature_t2>, <feature_t3>] and [1, 2, 3])
+
+    Usage Example:
+    - "Slope($close, %d)/$close"
+
+    # TODO:
+    # Some users may want pair-wise rolling like `Slope(A, B, N)`
 
     Parameters
     ----------