Add Core Web Vital Metrics to descriptions (#3454)

* FID to INP
* Add CLS description too
* Explain why metric is related to API

Co-authored-by: Daniel D. Beck <[email protected]>

Author: tunetheweb

features/event-timing.yml

@@ -1,5 +1,5 @@
 name: Event timing
-description: The `event` and `first-input` performance entries and the `PerformanceEventTiming` API measures the latency of user input events, such as mouse clicks or keypresses, and the latency of the first user input. First Input Delay (FID) is a common metric for perceived responsiveness.
+description: The `event` and `first-input` performance entries and the `PerformanceEventTiming` API measures the latency of user input events, such as mouse clicks or keypresses. They're used to calculate Interaction to Next Paint (INP), a common metric for perceived responsiveness.
 spec: https://w3c.github.io/event-timing/
 group: performance
 status:

features/layout-instability.yml

@@ -1,5 +1,5 @@
 name: Layout instability
-description: The `layout-shift` performance entry and `LayoutShift` API measures the layout stability of web pages based on movements of the elements on the page.
+description: The `layout-shift` performance entry and `LayoutShift` API measures the layout stability of web pages based on movements of the elements on the page. They're used to calculate Cumulative Layout Shift (CLS), a common metric for perceived stability.
 spec: https://wicg.github.io/layout-instability/
 group: performance
 compat_features: