Absolute value or minus sign missing in docstrings for head and tail
In the docstrings for the head and tail functions (in pandas/core/generic.py) there is an absolute value or minus sign missing:
e.g. in tail docstring
```
For negative values of `n`, this function returns all rows except
        the first `n` rows, equivalent to ``df[n:]``.
```

should read something like
```
For negative values of `n`, this function returns all rows except
        the first `np.abs(n)` rows, equivalent to ``df[np.abs(n):]``.
```

or 

```
For negative values of `n`, this function returns all rows except
        the first `-n` rows, equivalent to ``df[-n:]``.
```
Similarly for head.