API: which "anchor point" for datetime properties of Periods ?
In several docstring PRs for Period datetime properties, we ran into the confusion about how the date/time of those attributes are determined (start or end ?). Eg see discussion in https://github.com/pandas-dev/pandas/pull/20277#discussion_r173651389

Small example to illustrate:

```
In [30]: p1 = pd.Period('2017-01-01', freq='D')

In [31]: p2 = pd.Period('2017-01-01', freq='M')

In [32]: p1
Out[32]: Period('2017-01-01', 'D')

In [33]: p2
Out[33]: Period('2017-01', 'M')

In [34]: p1.start_time
Out[34]: Timestamp('2017-01-01 00:00:00')

In [35]: p2.start_time
Out[35]: Timestamp('2017-01-01 00:00:00')

In [36]: p1.day
Out[36]: 1

In [37]: p2.day
Out[37]: 31
```

The discussion raised from how to describe the summary of such an attribute: "The day of the month" -> but which day of the period span? -> should this be "The day of the month of the start of the Period" ? -> ah, no, because it is not always the start, it depends on the frequency.

In the above example, `M` is actually the freq string for "MonthEnd", and the datetime-properties apparently then use the end as date to calculate those properties.

Questions:

- How best to document this? Can we use a certain phrase in all docstrings?
- Is there a way to know, given a certain `freq`, what the "anchor point" is? (using anchor point here, don't know if we have existing terminology for that) A way to know if the `freq` is a "<some-freq>End" ?
- It's rather confusing behaviour, is this actually the behaviour we want?

cc @jreback @jbrockmendel @sinhrks 