Note use of -- for -i to behave like cpython

Providing script arguments is only really useful with -i, and while this is described in the main Usage section of ipython --help, it seems more directly helpful here

IPython/terminal/ipapp.py

@@ -156,7 +156,10 b" frontend_flags['quick']=("
 
 frontend_flags['i'] = (
     {'TerminalIPythonApp' : {'force_interact' : True}},
-    """If running code from the command line, become interactive afterwards."""
+    """If running code from the command line, become interactive afterwards.
+    It is often useful to follow this with `--` to treat remaining flags as
+    script arguments.
+    """
 )
 flags.update(frontend_flags)