Document Expected Interface IP Allocation Policies

[chet]

Update the generated CLI and operator-facing documentation for the ExpectedMachine interface roles and IP allocation policies added by #3990. Keep this work in a docs-only PR so technical-writing review does not block the code change.

What this involves

• Update or regenerate the docs/ CLI reference for role, ip_allocation, fixed_ip, and the optional network_segment_type guard.
• Explain Dynamic, Fixed, and Retained behavior for the currently supported Host, DpuOs, and DpuBmc roles.
• Explain legacy inference when newer fields are omitted: fixed_ip selects Fixed; otherwise allocation remains Dynamic.
• Clarify that fixed addresses select a segment by prefix containment, while DHCP uses the relay or link segment and treats network_segment_type as an optional check.
• Keep HostBmc, DPU loopbacks, and the ExpectedInterface rename out of this update until their implementation issues land.
• Use a dedicated docs worktree and docs-only PR that references Support IP allocation policies for expected interfaces #3990 and feat: support IP allocation policies for expected interfaces #3991.

Part of #3491.