Adding info about combining css classes (#3329)

* Adding info about combining css classes 

Adding info about combining css classes using comma separation

* Update 05_Configuration.md

* Apply suggestions from code review

Co-authored-by: Frans de Jonge <fransdejonge@gmail.com>

Co-authored-by: Frans de Jonge <fransdejonge@gmail.com>

docs/en/users/05_Configuration.md

@@ -182,6 +182,22 @@ Here we find that the block that encompasses nothing but the content of the arti
 *  Lesnumériques: ```article#body div.text.clearfix```
 *  Phoronix: ```#main .content```
 
+#### Combining CSS Classes
+Let's say we have an article which contains ads within its content, and we do not want to have those ads retrieved by FreshRSS. Example Source Code:
+```
+<div id='article'>
+<h2>wanted</h2>
+<p class='content'>wanted content</p>
+<p class='ad'>unwanted content</p>
+<h2>wanted</h2>
+<p class='content'>wanted content</p>
+<h2>wanted</h2>
+<p class='ad'>unwanted content</p>
+<p class='content'>wanted content</p>
+</div>
+```
+In this case it's possible to combine multiple CSS selectors with a comma: ```#article p.content, #article h2```
+
 ### Retrieve a truncated stream with external tools
 
 Complimentary tools can be used to retrieve full article content, such as: