Add a docstring to the IncrementalTransform class (#1459)

dlt/extract/incremental/transform.py

@@ -34,6 +34,15 @@
 
 
 class IncrementalTransform:
+    """A base class for handling extraction and stateful tracking
+    of incremental data from input data items.
+
+    By default, the descendant classes are instantiated within the
+    `dlt.extract.incremental.Incremental` class.
+
+    Subclasses must implement the `__call__` method which will be called
+    for each data item in the extracted data.
+    """
     def __init__(
         self,
         resource_name: str,
@@ -100,6 +109,7 @@ def deduplication_disabled(self) -> bool:
 
 
 class JsonIncremental(IncrementalTransform):
+    """Extracts incremental data from JSON data items."""
     def find_cursor_value(self, row: TDataItem) -> Any:
         """Finds value in row at cursor defined by self.cursor_path.