Correct documentation, nodes classes do not exend python's ones (#1343)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: tristanlatr

astroid/__init__.py

@@ -22,8 +22,11 @@
 pylint... Well, actually the development of this library is essentially
 governed by pylint's needs.
 
-It extends class defined in the python's _ast module with some
-additional methods and attributes. Instance attributes are added by a
+It mimics the class defined in the python's _ast module with some
+additional methods and attributes. New nodes instances are not fully
+compatible with python's _ast.
+
+Instance attributes are added by a
 builder object, which can either generate extended ast (let's call
 them astroid ;) by visiting an existent ast tree or by inspecting living
 object. Methods are added by monkey patching ast classes.