update v8ForceOptimization docs

Author: joyeecheung

benchmark/README.md

@@ -235,12 +235,12 @@ it returns to accomplish what they need. This function reports timing
 data to the parent process (usually created by running `compare.js`, `run.js` or
 `scatter.js`).
 
-### v8ForceOptimization(method)
+### v8ForceOptimization(method[, ...args])
 
-Force V8 to optimize the function with the native function
+Force V8 to mark the `method` for optimization with the native function
 `%OptimizeFunctionOnNextCall()` and return the optimization status
 after that.
 
-It is used to prevent the benchmark from getting disrupted
-by the optimizer kicking in halfway through. However, this could result in a
-less effective optimization, so it should be used with caution.
+It can be used to prevent the benchmark from getting disrupted by the optimizer
+kicking in halfway through. However, this could result in a less effective
+optimization. In general, only use it if you know what it actually does.