Fix #10770 by adding days_in_month to docs
Fix https://github.com/pydata/pandas/issues/10770 by adding days_in_month to Time/Date Components. Am I right in thinking `days_in_month` (rather than `daysinmonth`) is the right name to use going forward? Is there anywhere else in the documentation that it should be added that I missed?

I think this is good! 
In the API reference it is already added: http://pandas.pydata.org/pandas-docs/stable/api.html#datetimelike-properties
 thanks!

interested in working on #9606 ?
 Yeah, sure!
