Deprecate %autoindent

Closes gh-9888

IPython/terminal/magics.py

@@ -81,8 +81,9 b' class TerminalMagics(Magics):'
 
     @line_magic
     def autoindent(self, parameter_s = ''):
-        """Toggle autoindent on/off (if available)."""
-
+        """Toggle autoindent on/off (deprecated)"""
+        print("%autoindent is deprecated: you can now paste multiple lines "
+              "without turning autoindentation off.")
         self.shell.set_autoindent()
         print("Automatic indentation is:",['OFF','ON'][self.shell.autoindent])