docs(estimatedDocumentCount): Documentation for countDocuments mentions estimatedDocumentCount (#2020)
docs(estimatedDocumentCount): Documentation for countDocuments mentions estimatedDocumentCount (#2020)

* Documentation for countDocuments MUST mention estimatedDocumentCount
* mentions estimatedDocumentCount in documentation for countDocuments and made estimatedDocumentCount a link.

Fixes NODE-1981

mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native