Update the cp command docs according to commit d40a17ffc2

[boaz0]

related to #30877 and #28923

- What I did

Update the docs

- How I did it

Add a small paragraph regarding the ownership of the copied files.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "cp_permissions_docs" git@github.com:ripcurld0/moby.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[thaJeztah]

Thanks! I think this needs a mention of the new flag that was added (which makes the actual change);

-a, --archive       Archive mode (copy all uid/gid information)

[cpuguy83]

Thanks @ripcurld0.

These docs were moved to github.com/docker/cli. Can you open this over there?
Closing this one since we can't merge here.