DOC: read_excel dtype parameter - str vs. object 
#### Code Sample, a copy-pastable example if possible

```ipython
In [16]: import pandas as pd
    ...: import numpy as np
    ...: pd.DataFrame(['a', 1, np.nan]).to_excel('test.xlsx')
    ...: df_str = pd.read_excel('test.xlsx', dtype=str, names=['col_str'])
    ...: df_str['type_str'] = df_str.col_str.map(type)
    ...: df_obj = pd.read_excel('test.xlsx', dtype=object, names=['col_obj'])
    ...: df_obj['type_obj'] = df_obj.col_obj.map(type)
    ...: pd.concat([df_str, df_obj], axis=1)
    ...: 
Out[16]: 
  col_str       type_str col_obj         type_obj
0       a  <class 'str'>       a    <class 'str'>
1       1  <class 'str'>       1    <class 'int'>
2     nan  <class 'str'>     NaN  <class 'float'>
```
#### Problem description

I imagine <a href="http://pandas.pydata.org/pandas-docs/version/0.20/generated/pandas.read_excel.html">read_excel</a>'s `dtype` parameter description should just read "Use *object* to preserve and not interpret dtype" and not "Use *str* or *object*".