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).
