Bug when retrieving docstring with __getitem__

python 3.6.4
IPython 6.2.1

```python
class Foo(object):
def __getitem__(self, key):
return key**2

f = Foo()
f[2].clear??
```

Returns the following docstring
```
Source:
@line_magic
def clear(self, arg_s):
"""Clear the terminal."""
if os.name == 'posix':
self.shell.system("clear")
else:
self.shell.system("cls")
File: c:\\lib\site-packages\ipykernel\zmqshell.py
```

Reproduce the issue with the provided Foo class and the f[2].clear?? expression, then inspect how docstrings are retrieved around ipykernel/zmqshell.py. Done means the expression no longer returns the unrelated clear terminal docstring, though the issue does not identify a test or target source file.

Written by the indexing model from the issue text.