DOC: DatetimeIndex day says the same as month
#### Location of the documentation

https://pandas.pydata.org/docs/reference/api/pandas.DatetimeIndex.day.html#pandas.DatetimeIndex.day

#### Documentation problem

Right now it says the same as month, `The month as of January=1, December=12`

#### Suggested fix for documentation

Something along the lines of `Day of the month, from 1 through 31` 
