Update the *hdfs docs about streamed copy

[isidentical]

See: https://github.com/intake/filesystem_spec/pull/762#issuecomment-923967545

[martindurant]

Short summary: some implementations don't support a native copy operation, but we can and do emulate it by reading the file and writing to a new remote file. This achieves the same thing, but involves steaming all the data through the local process, which may be surprising to users.

This issue is to discuss if/where we should let users know.