docs: clarify container machine vs container semantics

[Bortlesboat]

Summary

• clarify that container machines use standard OCI images to provide long-lived Linux environments
• explain that both regular containers and container machines persist changes until deletion
• frame the distinction around reusable environment workflows and host user/home integration

Verification

• git diff --check
• reviewed the updated docs/container-machine.md wording against issue [Request]: strange doc comments about machines versus containers #1700 and maintainer comments

Closes #1700

[afbjorklund]

emphasize the real differentiators: dedicated persistent root storage, init/system-service support,

I don't think the first two are real differentiators, but the new wording makes the intent clear enough.

[afbjorklund]

a container machine is a convenience wrapper around a Linux environment, not a wholly separate runtime model

Not necessarily true, currently container ls and container machine ls are totally separated runtime models...