Can't find any documentation about Object.bsonsize()

I try to use Object.bsonsize(), but got below error:

Atlas atlas-zy5qpa-shard-0 [primary] test> Object.bsonsize(user)
TypeError: Object.bsonsize is not a function 

I found this answer: Object.bsonsize is not a function - #2 by Anna_Henningsen

But where can I find the documentation about the legacy Object.bsonsize() and the bsonsize() methods? I search the documentation but didn’t find anything about them.