IntervalIndex.get_indexer "method" options are not implemented but documentation implies they are
#### Problem description

`IntervalIndex.get_indexer` returns `NotImplementedError` if `method` is any of the ones shown in the docstring and documentation (except for 'method=None'). I understand that they are not implemented yet, but that is not clear from the docstring.  It presents the 5 available options as if they are implemented. 

Wouldn't it be better to remove `method`, if none of the options are available? Or at least make it more clear in the documentation. 

Documentation: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.IntervalIndex.get_indexer.html

Exception is raised here:
https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/interval.py#L811
