DOC: update the PeriodIndex docstring
year, month, quarter, day, hour, minute, second documentation
improved. Detailed in depth the requirements of each parameter.

Documentation is still confusing for allowable freq arguments

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [X] PR title is "DOC: update the <your-function-or-method> docstring"
- [X] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [X] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [X] It has been proofread on language by another sprint participant

```
################################################################################
################################## Validation ##################################
################################################################################

Errors found:
	Errors in parameters section
		Parameters {'fields', 'name', 'ordinal'} not documented
		Unknown parameters {'second', 'month', 'year', 'minute', 'quarter', 'day', 'hour'}
		Parameter "dtype" has no description
	No returns section found
```

A few errors are still reported but are outside of the scope of the doc sprint (fields, name, ordinal).

Hello @costrouc! Thanks for submitting the PR.

 - In the file [`pandas/core/indexes/period.py`](https://github.com/pandas-dev/pandas/blob/594708a909c952fe2b772fee99e07104b42c38b5/pandas/core/indexes/period.py), following are the PEP8 issues :

> [Line 212:80](https://github.com/pandas-dev/pandas/blob/594708a909c952fe2b772fee99e07104b42c38b5/pandas/core/indexes/period.py#L212): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (87 > 79 characters)

 Ok so I am sort of lost with all the feedback. 

I hear the requests for changes but many of these parameters require multiple sentences to describe. The `PeriodIndex` class has three ways of initializing each one requiring a subset of the arguments. The `_generate_range` and `_range_from_fields` methods of the class show this. Here are the three ways:

 1. `PeriodIndex(start, end, periods, freq)`
 2. `PeriodIndex(year, quarter, freq)`
 3. `PeriodIndex(year, month, day, hour, minute, second, freq)`

Case 2 requires that the freq must match Q or Q-DEC.

Additionally while year, month, day, hour, minute and second all do default to None for cases 1 and 2. Case 3 has a completely different default behavior (day=1, hour=0, minute=0, second=0) while year and month are required. Thanks for listing out the cases. I think as long as each of those has at least one example we're OK. closing as stale. if you'd like to continue pls ping.