DOC: Add missing docstrings
Added docstrings:
```
pandas.Index.has_duplicates
pandas.Index.is_all_dates
pandas.Index.name
pandas.Index.is_boolean
pandas.Index.is_floating
pandas.Index.is_integer
pandas.Index.is_interval
pandas.Index.is_mixed
pandas.Index.is_numeric
pandas.Index.is_object
```

There are some other docstrings left - I couldn't find the docstrings below in the code:
```
None:None:GL08:pandas.Index.names:The object does not have a docstring
None:None:GL08:pandas.Index.empty:The object does not have a docstring
```

I don't really understand what the function does so I'm leaving the docstring empty for now:
```
pandas/pandas/core/indexes/base.py:639:GL08:pandas.Index.view:The object does not have a docstring
```

cc @datapythonista

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
