Docs for SERVER-22949: Remove default limit from $geoNear aggregation stage

Description:

Marking docs changes as needed because the default limit (of 100 results) is gone. If you would like to limit results, use a $limit stage in the equivalent aggregation.

Engineering Ticket Description:

The default limit for the $near/$nearSphere query operators was removed in SERVER-14515. For consistency, the default limit should also be removed from the $geoNear aggregation stage.