DOC: Fix RangeIndex and other docstrings for missing period in summary
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@datapythonista 

I added the period to the following cases:
pandas.Categorical.dtype: Summary does not end with a period
pandas.merge_ordered: Summary does not end with a period
pandas.bdate_range: Summary does not end with a period
pandas.period_range: Summary does not end with a period
pandas.timedelta_range: Summary does not end with a period
pandas.interval_range: Summary does not end with a period
pandas.util.hash_pandas_object: Summary does not end with a period
pandas.Grouper: Summary does not end with a period
pandas.Index.memory_usage: Summary does not end with a period
pandas.Index.fillna: Summary does not end with a period
pandas.Index.dropna: Summary does not end with a period
pandas.RangeIndex.start: Summary does not end with a period
pandas.RangeIndex.stop: Summary does not end with a period
pandas.RangeIndex.step: Summary does not end with a period