Add DATE_RANGE_MILLIS to the date functions documentation, also update the description for DATE_RANGE_STR.

Please look at the date design documentation.
https://docs.google.com/document/d/1CkGGi6gyFpBIcRkM5W_kkHd-r5GxI-bZvW1new8aBhA/edit?usp=sharing

There is a full description for the DATE_RANGE_MILLIS.

For DATE_RANGE_STR, now the result array will not contain the end date. Also we have removed the ARRAY_DATE_RANGE alias.

Both of these is tracked by the following MB's
MB-21413
MB-21745