feat(webvitals): Performance scores are only supported on desktop web browsers (#9076)

Adds new line: "Performance Scores are currently only supported on desktop web browsers" to performance score doc

Author: edwardgou-sentry

docs/product/performance/web-vitals.mdx

@@ -147,7 +147,7 @@ The **Performance Score** is comprised of 5 individual **Web Vital** components.
 | [First Contentful Paint](#first-contentful-paint-fcp) (FCP)     | [900ms](https://www.desmos.com/calculator/gcxbiypuuh)  | [1600ms](https://www.desmos.com/calculator/gcxbiypuuh) | 15%    |
 | [Time To First Byte](#time-to-first-byte-ttfb) (TTFB)           | [200ms](https://www.desmos.com/calculator/ykzahw9goi)  | [400ms](https://www.desmos.com/calculator/ykzahw9goi)  | 10%    |
 
-Find out which Web Vitals are required to calculate Performance Scores in the [Browser Support](#browser-support) table.
+Performance Scores are currently only supported on desktop web browsers. Find out which Web Vitals are required to calculate Performance Scores in the [Browser Support](#browser-support) table.
 
 ## Opportunity