Auto merge of #20626 - avadacatavra:pwm-spec, r=asajeffrey

added spec comments for progressive web metrics

<!-- Please describe your changes on the following line: -->
r? @asajeffrey

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach build-geckolib` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #20624 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it's a comment

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20626)
<!-- Reviewable:end -->

components/metrics/lib.rs

@@ -102,6 +102,7 @@ fn set_metric<U: ProgressiveWebMetric>(
 
 }
 
+// spec: https://github.com/WICG/time-to-interactive
 // https://github.com/GoogleChrome/lighthouse/issues/27
 // we can look at three different metrics here:
 // navigation start -> visually ready (dom content loaded)
@@ -259,6 +260,7 @@ impl ProgressiveWebMetric for InteractiveMetrics {
     }
 }
 
+// https://w3c.github.io/paint-timing/
 pub struct PaintTimeMetrics {
     pending_metrics: RefCell<HashMap<Epoch, (Option<TimerMetadata>, bool)>>,
     navigation_start: Option<u64>,