cpu: comment describing fields.

src/widgets/cpu.rs

@@ -50,6 +50,10 @@ impl CpuWidget {
                     .iter()
                     .map(|x| u64::from_str_radix(x, 10).unwrap())
                     .collect::<Vec<_>>();
+                // [0] user
+                // [1] nice
+                // [2] system
+                // [3] idle
                 let usage = parsed[0] + parsed[1] + parsed[2];
                 let total = usage + parsed[3];
                 stats.push(Stat { usage, total })