docs: clarify container machine vs container semantics

[Bortlesboat]

Summary

• clarify that a container machine is a convenience wrapper around a Linux environment, not a wholly separate runtime model
• explain that both regular containers and container machines persist changes until deletion
• emphasize the real differentiators: dedicated persistent root storage, init/system-service support, 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