Add WaitForFunctionAsync doc (#1504)

Author: kblok

docfx_project/examples/Page.WaitForFunctionAsync.md

@@ -0,0 +1,30 @@
+# How to wait for a javascript expression to be true
+_Contributors: [Darío Kondratiuk](https://www.hardkoded.com)_
+
+## Problem
+
+`WaitForSelectorAsync` is not enough, you want to wait for a more complex javascript expression to be truthly.
+
+## Solution
+
+Use [Page.WaitForExpressionAsync](https://www.puppeteersharp.com/api/PuppeteerSharp.Page.html#PuppeteerSharp_Page_WaitForExpressionAsync_System_String_PuppeteerSharp_WaitForFunctionOptions_) or [Page.WaitForFunctionAsync](https://www.puppeteersharp.com/api/PuppeteerSharp.Page.html#PuppeteerSharp_Page_WaitForFunctionAsync_System_String_PuppeteerSharp_WaitForFunctionOptions_System_Object___) to delay execution until the result of a javascription expression is truthly.
+
+If it's a simple expression you can use `WaitForFunctionAsync`:
+
+```cs
+using (var browser = await Puppeteer.LaunchAsync(options))
+using (var page = await browser.NewPageAsync())
+{
+    await page.GoToAsync("https://www.somepage.com");
+    await Page.WaitForExpressionAsync("document.queryselector('#status_info').innerText.match('^Showing ([1-9][0-9]*?) to ([1-9][0-9]*?)') of ([1-9][0-9]*?) entries') != null");
+} 
+```
+
+If the evaluation is more complex, you could wrap it inside a function and use `WaitForFunctionAsync`:
+
+```cs
+var waitTask = Page.WaitForFunctionAsync(@"() => 
+{
+    return document.queryselector('#status_info').innerText.match('^Showing ([1-9][0-9]*?) to ([1-9][0-9]*?)') of ([1-9][0-9]*?) entries') != null;
+}");
+```

docfx_project/examples/toc.yml

@@ -12,6 +12,8 @@
       href: ReuseChrome.md
     - name: How to test a Chrome Extension
       href: ChromeExtension.md
+    - name: How to wait for a javascript expression to be true
+    - href: Page.WaitForFunctionAsync.md
 - name: Advanced
   items:
     - name: How to log CDP communication