feat(visibility): Add a note about apdex score (#5081)

- This is cause I've heard folks confused whether a higher apdex score
  is better or not, and personally forget sometimes.

Author: wmak

src/docs/product/performance/metrics.mdx

@@ -10,7 +10,7 @@ Metrics provide insight about how users are experiencing your application. In [P
 
 ## Apdex
 
-Apdex is an industry-standard metric used to track and measure user satisfaction based on your application response times. The Apdex score provides the ratio of satisfactory, tolerable, and frustrated requests in a specific transaction or endpoint. This metric provides a standard for you to compare transaction performance, understand which ones may require additional optimization or investigation, and set targets or goals for performance.
+Apdex is an industry-standard metric used to track and measure user satisfaction based on your application response times. A higher Apdex score is better than a lower one; the score can go up to 1.0, representing 100% of users having a satisfactory experience. The Apdex score provides the ratio of satisfactory, tolerable, and frustrated requests in a specific transaction or endpoint. This metric provides a standard for you to compare transaction performance, understand which ones may require additional optimization or investigation, and set targets or goals for performance.
 
 Below are the components of Apdex and its formula: