Update date range documentation to expose support for 4-digit absolute year ranges

Documentation for "Years back and forward" on the date widget only suggests a relative range like "-3:+3" but supports hidden functionality for a 4-digit absolute range like "1999:2010" or a combination of relative and absolute like "1999:+3".

This patch updates documentation to expose those options to the user.
Only applies to 6.x branch -- 7.x branch splits range values into 2 fields and documents the 4-digit absolute option.

Note that 4-digit relative ranges are not currently supported (see #1335578: date ranges fail for any range of 4+ digits - see date_range_years()) but this documentation patch assumes that lack of support is a bug and does not indicate this limitation.