Fix incorrect documentation for docker cp -a flag

[zozo123]

Summary

• Fixed the -a (--archive) flag description for docker cp which incorrectly stated "Archive mode (copy all uid/gid information)"
• Updated the extended documentation in container_cp.md which incorrectly described the default and -a behaviors
• Updated fish shell completions to match

Problem

The documentation claimed -a means "copy all uid/gid information", implying it preserves original uid/gid. In practice:

• Without -a (default): docker cp already preserves ownership (uid/gid) from the source
• With -a: ownership is changed to match the container user (as configured with --user)

The old docs described the opposite of what actually happens. See also moby/moby#34096 for the related behavior discussion.

Changes

File	Change
cli/command/container/cp.go	Updated Go flag description string
docs/reference/commandline/cp.md	Updated options table description
docs/reference/commandline/container_cp.md	Updated options table + extended description paragraph
contrib/completion/fish/docker.fish	Updated fish shell completion description

Test plan

• Verified all instances of old text are replaced (grep returns no matches for "copy all uid/gid information")
• Reviewed diff to confirm only documentation strings changed, no logic changes
• Documentation-only change; no functional tests affected

Closes #6870