Document the output fields for currentOp when run on a mongos

There is no documentation for what the output fields of currentOp are when run on a mongos, but there is documentation for a mongod: https://docs.mongodb.com/manual/reference/command/currentOp/#currentop-output-fields

The output of currentOp on a mongos is very different than the output on a mongod. The output also varies between different version of MongoDB. It would be helpful for users to understand what information is available, particularly because the server does not support killing queries run on sharded clusters. This is a pain point for many users. See SERVER-6496.

New fields were added in 3.6, in particular command.$client.mongos.client, that allow users to locate the shard and operation IDs for commands originating from a client connection by looking at the host and port.