Improve documentation for "Ignore datasets older than" filter

[wettenhj]

The "Ignore datasets older than" filter uses the dataset folder's Created date.

This is not sufficiently clear to users. We need to clarify the difference between Created and Modified dates and ensure that users understand that MyData will ignore new files added to old dataset folders when this filter is turned on.

See: http://mydata.readthedocs.io/en/latest/settings.html#filters
and https://github.com/mytardis/mydata/blob/develop/docs/source/settings.rst#filters