Add documentation for when live migration is multithreaded

[DorBreger]

What this PR does / why we need it:
this explicitly documents that VMs with explicitly defined cpu limits (not ones created by the autocpulimits feature) will not go through parallel live migration as per this line of code .

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[jean-edouard]

/approve

[kubevirt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jean-edouard

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• docs/compute/OWNERS [jean-edouard]

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[jean-edouard]

Suggested change

	For virtual machines that explicitly set resource limits, multithreaded migration is disabled and single-threaded migration will be used instead.
	For virtual machines that explicitly set CPU resource limits, multithreaded migration is disabled and single-threaded migration will be used instead.