docs(API): add more explicit examples for page.evaluate (#1711)

docs/api.md

@@ -680,11 +680,12 @@ List of all available devices is available in the source code: [DeviceDescriptor
 If the function, passed to the `page.evaluate`, returns a [Promise], then `page.evaluate` would wait for the promise to resolve and return its value.
 
 If the function passed into `page.evaluate` returns a non-[Serializable] value, then `page.evaluate` resolves to `undefined`.
+Passing arguments to ```pageFunction```.
 
 ```js
-const result = await page.evaluate(() => {
-  return Promise.resolve(8 * 7);
-});
+const result = await page.evaluate(x => {
+  return Promise.resolve(8 * x);
+}, 7);
 console.log(result); // prints "56"
 ```
 
@@ -692,6 +693,8 @@ A string can also be passed in instead of a function.
 
 ```js
 console.log(await page.evaluate('1 + 2')); // prints "3"
+const x = 10;
+console.log(await page.evaluate(`1 + ${x}`)); // prints "11"
 ```
 
 [ElementHandle] instances can be passed as arguments to the `page.evaluate`: