GODRIVER-572 document that Client, Collection, and Database methods will return an ErrNilDocument if nil is passed in for an interface{}

Change-Id: I1dd743891dd2ca49ebee0b76fa15f5ea738813a4

Author: Isabella Siu

mongo/doc.go

@@ -55,6 +55,9 @@
 //    if err != nil { return err }
 //    // do something with result...
 //
+// All Client, Collection, and Database methods that take parameters of type interface{}
+// will return ErrNilDocument if nil is passed in for an interface{}.
+//
 // Additional examples can be found under the examples directory in the driver's repository and
 // on the MongoDB website.
 package mongo