Document visit method return type (#13831)

Author: srittau

stdlib/ast.pyi

@@ -1893,8 +1893,12 @@ if sys.version_info >= (3, 14):
     def compare(left: AST, right: AST, /, *, compare_attributes: bool = False) -> bool: ...
 
 class NodeVisitor:
+    # All visit methods below can be overwritten by subclasses and return an
+    # arbitrary value, which is passed to the caller.
     def visit(self, node: AST) -> Any: ...
     def generic_visit(self, node: AST) -> Any: ...
+    # The following visit methods are not defined on NodeVisitor, but can
+    # be implemented by subclasses and are called during a visit if defined.
     def visit_Module(self, node: Module) -> Any: ...
     def visit_Interactive(self, node: Interactive) -> Any: ...
     def visit_Expression(self, node: Expression) -> Any: ...