improve descriptions

Author: s7tya

site/frontend/src/pages/dashboard.ts

@@ -89,7 +89,7 @@ function renderRuntime(element: string, data: [number], versions: [string]) {
       type: "line",
     },
     title: {
-      text: `Average time for a run`,
+      text: `Average time for a runtime benchmark`,
     },
     yAxis: {
       title: {text: "Seconds"},

site/frontend/templates/pages/dashboard.html

@@ -19,7 +19,9 @@
     <code>1.28.0</code>, along with the latest <code>beta</code> release. The displayed
     duration is an arithmetic mean amongst all
     <a href="https://github.com/rust-lang/rustc-perf/tree/master/collector/compile-benchmarks#stable">stable</a>
-    benchmarks.
+    benchmarks. The dashboard also shows the average duration of runtime benchmarks, which measure the performance of
+    Rust programs
+    compiled by a given version of the Rust compiler.
 </details>
 
 <div class="graphs">