Document ISODate and other extended JSON support directly in mongoimport docs

This ticket is an extension of DOCS-2539, but specifically focused on the mongoimport documentation rather than the extended JSON page here: http://docs.mongodb.org/manual/reference/mongodb-extended-json/

I've heard comments that in general having more documentation of this extended JSON format directly in the mongoimport docs would be helpful, as users skimming through may not realize the significance of the extended JSON page.