ENH: Add 'ascending' parameter to series/searchsorted() method.
Hello,

i tried using [pandas.Series.searchsorted](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.searchsorted.html) method in series.

**Example-1** 
```
input:
import pandas as pd
ser_order = pd.Series([1, 1, 2,3,4])
ser_order.searchsorted([2,3])

output:
array([2,3]) #correct result.
```

**Example-2** 
```
input:
import pandas as pd
ser_order = pd.Series([2, 2,3,5,6,3,1])
ser_order.searchsorted([2,4])

output:
array([0,3]) #wrong result.
```
So  above two example clearly shows that [pandas.Series.searchsorted](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.searchsorted.html) will work only for ordered array.if we could add parameter **ascending**,then we can resolve this issue.

Thanks