PYTHON-1880: Raise a warning when no_cursor_timeout is used with an implicit session (#594)

(cherry picked from commit 1818553)

Author: Kn99HN

pymongo/cursor.py

@@ -147,6 +147,14 @@ def __init__(self, collection, filter=None, projection=None, skip=0,
         if not isinstance(limit, int):
             raise TypeError("limit must be an instance of int")
         validate_boolean("no_cursor_timeout", no_cursor_timeout)
+        if no_cursor_timeout and not self.__explicit_session:
+            warnings.warn("use an explicit session with no_cursor_timeout=True "
+                          "otherwise the cursor may still timeout after "
+                          "30 minutes, for more info see "
+                          "https://docs.mongodb.com/v4.4/reference/method/"
+                          "cursor.noCursorTimeout/"
+                          "#session-idle-timeout-overrides-nocursortimeout",
+                          UserWarning, stacklevel=2)
         if cursor_type not in (CursorType.NON_TAILABLE, CursorType.TAILABLE,
                                CursorType.TAILABLE_AWAIT, CursorType.EXHAUST):
             raise ValueError("not a valid value for cursor_type")