Manual: Clarify that nesting depth counts both objects and arrays

https://www.mongodb.com/docs/manual/reference/limits/#mongodb-limit-Nested-Depth-for-BSON-Documents

As currently worded it seems to imply that there is no limit for array nesting. The intention is to count each layer of arrays and objects as part of a combined depth. CodeWithScope also pushes 2 layers, although I doubt it is worth mentioning that since it is an obscure and deprecated type.