[docs] Enhancements for client scripts (#4110)

* Fixes for client script docs

* Address remarks

docs/articles/documentation/using-testcafe/common-concepts/inject-scripts-into-tested-pages.md

@@ -7,11 +7,22 @@ permalink: /documentation/using-testcafe/common-concepts/inject-scripts-into-tes
 
 TestCafe allows you to inject custom client scripts into pages visited during the tests. You can add scripts that mock browser API or provide helper functions.
 
+* [Add Client Scripts to All Tests](#add-client-scripts-to-all-tests)
+* [Add Client Scripts to Specific Tests](#add-client-scripts-to-specific-tests)
+* [Provide Scripts to Inject](#provide-scripts-to-inject)
+  * [Inject a JavaScript File](#inject-a-javascript-file)
+  * [Inject a Module](#inject-a-module)
+  * [Inject Script Code](#inject-script-code)
+  * [Provide Scripts for Specific Pages](#provide-scripts-for-specific-pages)
+  * [Inject Scripts Into Iframes](#inject-scripts-into-iframes)
+  * [Specify Multiple Scripts](#specify-multiple-scripts)
+* [Access DOM in the Injected Scripts](#access-dom-in-the-injected-scripts)
+
 Use [test run options](#add-client-scripts-to-all-tests) to add client scripts to all tests, or [test API](#add-client-scripts-to-specific-tests) to add them to specific fixtures or tests.
 
 ## Add Client Scripts to All Tests
 
-Use either of the following to inject scripts into pages visited during all the tests:
+Use either of the following options to inject scripts into pages visited during all the tests:
 
 * the [--cs (--client-scripts)](../command-line-interface.md#--cs-pathpath2---client-scripts-pathpath2) command line option
 
@@ -57,7 +68,7 @@ You can pass the following arguments to specify the scripts to inject:
 * [module name](#inject-a-module)
 * [script code](#inject-script-code)
 
-You can also [inject scripts into specific pages](#provide-scripts-for-specific-pages).
+You can also [inject scripts into specific pages](#provide-scripts-for-specific-pages) and [iframes](#inject-scripts-into-iframes).
 
 > Note that the API methods and configuration option support [multiple arguments](#specify-multiple-scripts).
 
@@ -138,7 +149,7 @@ runner.clientScripts({ path: 'assets/jquery.js' });
 }
 ```
 
-> You cannot combine the `path`, [module](#inject-a-module) and [content](#inject-script-code) properties.
+> You cannot combine the `path`, [module](#inject-a-module) and [content](#inject-script-code) properties in a single object. To inject multiple items, pass several arguments or an array.
 
 ### Inject a Module
 
@@ -206,7 +217,7 @@ fixture `My fixture`
 }
 ```
 
-> You cannot combine the `module`, [path](#inject-a-javascript-file) and [content](#inject-script-code) properties.
+> You cannot combine the `module`, [path](#inject-a-javascript-file) and [content](#inject-script-code) properties in a single object. To inject multiple items, pass several arguments or an array.
 
 ### Inject Script Code
 
@@ -271,7 +282,7 @@ test
 }
 ```
 
-> You cannot combine the `content`, [path](#inject-a-javascript-file) and [module](#inject-a-module) properties.
+> You cannot combine the `content`, [path](#inject-a-javascript-file) and [module](#inject-a-module) properties in a single object. To inject multiple items, pass several arguments or an array.
 
 ### Provide Scripts for Specific Pages
 
@@ -385,6 +396,17 @@ runner.clientScripts({
 
 > If the target page redirects to a different URL, ensure that the `page` property matches the destination URL. Otherwise, scripts are not injected.
 
+### Inject Scripts Into Iframes
+
+To inject a script into an iframe, specify the iframe URL in the [page](#provide-scripts-for-specific-pages) property.
+
+```js
+runner.clientScripts({
+    path: 'scripts/helpers.js',
+    page: 'https://example.com/iframe/'
+}));
+```
+
 ### Specify Multiple Scripts
 
 You can pass multiple arguments or an array to the `clientScripts` methods: