docs: adds Model#count to list of fns callback removed from

Author: SethFalco

docs/migrating_to_7.md

@@ -106,6 +106,7 @@ They always return promises.
 * `Model.aggregate`
 * `Model.bulkWrite`
 * `Model.cleanIndexes`
+* `Model.count`
 * `Model.countDocuments`
 * `Model.create`
 * `Model.createCollection`
@@ -130,6 +131,7 @@ They always return promises.
 * `Model.syncIndexes`
 * `Model.updateMany`
 * `Model.updateOne`
+* `Query.prototype.count`
 * `Query.prototype.find`
 * `Query.prototype.findOne`
 * `Query.prototype.findOneAndDelete`