DOC: Fix docstrings lack of punctuation
- [ ] xref #27977
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Similar to issue #27979, and as pointed out by @datapythonista, the summaries of some dosctrings don’t end with a period. 

I added the period to the following cases:
```
•       pandas.IndexSlice
•	pandas.MultiIndex.names
•	pandas.MultiIndex.is_lexsorted
•	pandas.MultiIndex.reorder_levels
•	pandas.DatetimeIndex.snap
•	pandas.DatetimeIndex.to_perioddelta
•	pandas.DatetimeIndex.to_pydatetime
•	pandas.DatetimeIndex.to_series
•	pandas.TimedeltaIndex
•	pandas.PeriodIndex.is_leap_year
•	pandas.api.extensions.ExtensionArray._concat_same_type
•	pandas.api.extensions.ExtensionArray.dropna
```

These are the outputs of validate_docstrings.py when evaluating each case:

**pandas.IndexSlice**
```
4 Errors found:
	Examples do not pass tests
	flake8 error: E231 missing whitespace after ',' (4 times)
	flake8 error: E902 TokenError: EOF in multi-line statement
	flake8 error: E999 SyntaxError: invalid syntax
1 Warnings found:
	No extended summary found
```
**pandas.MultiIndex.names**
```
3 Warnings found:
	No extended summary found
	See Also section not found
	No examples section found
```
**pandas.MultiIndex.is_lexsorted**
```
1 Errors found:
	Return value has no description
3 Warnings found:
	No extended summary found
	See Also section not found
	No examples section found
```
**pandas.MultiIndex.reorder_levels**
```
2 Errors found:
	Parameters {order} not documented
	Return value has no description
3 Warnings found:
	No extended summary found
	See Also section not found
	No examples section found
```
**pandas.DatetimeIndex.snap**
```
2 Errors found:
	Parameters {freq} not documented
	Return value has no description
3 Warnings found:
	No extended summary found
	See Also section not found
	No examples section found
```
**pandas.DatetimeIndex.to_perioddelta**
```
5 Errors found:
	Summary should fit in a single line
	Parameters {**kwargs, *args} not documented
	Unknown parameters {freq}
	Parameter "freq" has no description
	Return value has no description
2 Warnings found:
	See Also section not found
	No examples section found
```
**pandas.DatetimeIndex.to_pydatetime**
```
4 Errors found:
	Summary should fit in a single line
	Parameters {**kwargs, *args} not documented
	The first line of the Returns section should contain only the type, unless multiple values are being returned
	Return value has no description
2 Warnings found:
	See Also section not found
	No examples section found
```
**pandas.DatetimeIndex.to_series**
```
2 Errors found:
	Summary should fit in a single line
	Return value has no description
2 Warnings found:
	See Also section not found
	No examples section found
```
**pandas.TimedeltaIndex**
```
3 Errors found:
	Summary should fit in a single line
	Parameters {dtype, verify_integrity, periods, copy, data} not documented
	Unknown parameters {periods , copy , data }
1 Warnings found:
	No examples section found
```
**pandas.PeriodIndex.is_leap_year**
```
3 Warnings found:
	No extended summary found
	See Also section not found
	No examples section found
```
**pandas.api.extensions.ExtensionArray._concat_same_type**
```
2 Errors found:
	Parameter "to_concat" has no description
	Return value has no description
3 Warnings found:
	No extended summary found
	See Also section not found
	No examples section found
```
**pandas.api.extensions.ExtensionArray.dropna**
```
2 Errors found:
	The first line of the Returns section should contain only the type, unless multiple values are being returned
	Return value has no description
3 Warnings found:
	No extended summary found
	See Also section not found
	No examples section found
```
I would be happy to also work on the other errors/warnings in this PR on another.
@martinagvilas Thanks a lot!