resource: document overflow-safe arithmetic for device request Count

[suii2210]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Updates the GoDoc for ExactDeviceRequest.Count and DeviceSubRequest.Count across the internal and versioned resource APIs to clarify that:

• consumers should treat Count as untrusted input,
• aggregation and multiplication of Count values should use overflow-safe arithmetic, and
• consumers should not rely on unschedulability to protect accounting code.

This is a documentation-only change and does not modify validation or runtime behavior.

Which issue(s) this PR is related to:

Fixes #140424

Special notes for your reviewer:

The same documentation update has been applied consistently across the internal API and all maintained versioned resource API types.

Does this PR introduce a user-facing change?

NONE

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: suii2210
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