Change wording in perf summary PR comment to expose the used aggregation statistic

Author: Kobzol

site/src/comparison.rs

@@ -299,6 +299,11 @@ impl ComparisonSummary {
         self.average(self.improvements())
     }
 
+    /// The average change as a percent
+    pub fn average_change(&self) -> f64 {
+        self.average(&self.comparisons)
+    }
+
     /// The average regression as a percent
     pub fn average_regression(&self) -> f64 {
         self.average(self.regressions())
@@ -376,19 +381,27 @@ impl ComparisonSummary {
         if self.num_regressions > 1 {
             writeln!(
                 result,
-                "- Average relevant regression: {:.1}%",
+                "- Arithmetic mean of relevant regressions: {:.1}%",
                 self.average_regression()
             )
             .unwrap();
         }
         if self.num_improvements > 1 {
             writeln!(
                 result,
-                "- Average relevant improvement: {:.1}%",
+                "- Arithmetic mean of relevant improvements: {:.1}%",
                 self.average_improvement()
             )
             .unwrap();
         }
+        if self.num_improvements > 1 && self.num_regressions > 1 {
+            writeln!(
+                result,
+                "- Arithmetic mean of relevant changes: {:.1}%",
+                self.average_change()
+            )
+                .unwrap();
+        }
         for change in self.most_relevant_changes().iter().filter_map(|s| *s) {
             write!(result, "- ").unwrap();
             change.summary_line(result, link)