Fix calculateCPUPercent docstring parameter names and units

[anxkhn]

## Type of Change
- [ ] Bug fix
- [ ] New feature
- [ ] Breaking change
- [x] Documentation update

## Motivation and Context
The DocC docstring for `calculateCPUPercent` in
`Sources/ContainerCommands/Container/ContainerStats.swift` documented parameters
that no longer exist on the function. The `- Parameters:` block listed
`cpuUsageUsec1`, `cpuUsageUsec2`, and `timeDeltaUsec`, each described as "in
microseconds", but the actual signature is:

```swift
static func calculateCPUPercent(
    cpuUsage1: Duration,
    cpuUsage2: Duration,
    timeInterval: Duration
) -> Double

So all three documented names were stale and the "microseconds" unit contradicted
the Duration parameter types (the body divides Duration by Duration
directly). DocC / Xcode flag - Parameters: entries whose names do not match the
signature, so this produced parameter-name warnings and misleading docs.

This updates the docstring to the real parameter names and describes them as the
durations they are:

• cpuUsage1 / cpuUsage2: cumulative CPU time from each sample (the call site
  builds these with .microseconds(cpuUsageUsec) from the cumulative usage
  fields).
• timeInterval: the elapsed wall-clock time between the two samples (the call
  site passes .seconds(2), the sampling interval).

Comment-only change; no behavior change.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Doc-only change, so no unit test applies. Verified with the repo's tooling:

• swift format lint --strict --configuration .swift-format-nolint on the file:
  exit 0.
• swift format --configuration .swift-format on the file: no diff (already
  canonical).
• swift build --target ContainerCommands: builds cleanly; the file recompiles.