Shift-Tab completion doesn't work when edit cursor is at the start of the object

Shift-Tab completion on an object name only works when you place the edit cursor _inside_ or _after_ the object. Not when it's _at the start of the object_. I think that's contrary to most IDEs. Also, this gotcha is not documented anywhere.

```
import pandas
pd.crosstab
# ^ Put your edit cursor at the start of `pd.crosstab` and press Shift-Tab (on MacOS, Fn-Shift-Tab).
# Nothing happens.
```

Version: Jupyter 4.2.3 on MacOS

Reproduce the provided pandas/pd.crosstab example with the cursor at the start of the object and press Shift-Tab. Trace the Jupyter Notebook Shift-Tab completion behavior, then verify that completion works at the object start and that the gotcha is documented.

Written by the indexing model from the issue text.