Is it necessary to unify pandas and pd in docstring?
There are two kinds of demonstration of pandas in current docstring, `pandas.` and `pd.`.

For example, [`pd.`](https://github.com/pandas-dev/pandas/blob/aa3fefc898b2f101f3cf83f90add69857926fcbe/pandas/core/reshape/concat.py#L270):
```python
msg = ('cannot concatenate object of type "{0}";'
       ' only pd.Series, pd.DataFrame, and pd.Panel'
       ' (deprecated) objs are valid'.format(type(obj)))
```
and [`pandas.`](https://github.com/pandas-dev/pandas/blob/d59a7b5c4ab842399d79ffac120e9a46b4c0f8fa/pandas/json.py#L4):
```python
warnings.warn("The pandas.json module is deprecated and will be "
              "removed in a future version. Please import from "
              "pandas.io.json instead", FutureWarning, stacklevel=2)
```

`pd.` is friendly to experienced pandas users, but it may lead to misunderstanding for newbies(no offence).
So is it necessary to unify these two presentations?

I'd like to create a PR if it's necessary.