Clarify runtime cases

Author: Tyriar

.github/instructions/benchmark.instructions.md

@@ -11,6 +11,8 @@ applyTo: '**/*.benchmark.ts'
   - Use `-t` to get the path, then:
   - `npm run benchmark -- -s "<path>" out-test/benchmark/Event.benchmark.js`
 
+When writing instructions, use `RuntimeCase` to measure pure runtime in ms, use `ThroughputRuntimeCase` when measuring throughput in MB/s.
+
 Notes:
 - Benchmarks run from built JS in `out-test/benchmark/*.benchmark.js`.
 - Keep `NODE_PATH=./out` (handled by the npm script).