Update REFERENCE.md to include collector.dbstatsfreestorage

[knuteriksen]

Adds the --collector.dbstatsfreestorage option introduced in v.0.40.0.

I would also like to add a warning somewhere that states the same as the mongodb documentation:

If the instance has a large number of collections or indexes, obtaining free space usage data may cause processing delays

However, I am not sure where that would fit best.

MongoDB themselves recognized the potential impact to production by setting freeStorage to false by default here

Related pr introducing the option

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