Merge pull request magento#6968 from vasilii-b/event-observers-warning-about-unique-name

"Events and observers" page: Added warning about overriding observer by using existing name.

Author: shrielenee

src/guides/v2.3/extension-dev-guide/events-and-observers.md

@@ -127,6 +127,9 @@ The `observer` [xml](https://glossary.magento.com/xml) element has the following
 *  `disabled` - Determines whether this observer is active or not. Default value is false.
 *  `shared` - Determines the [lifestyle]({{ page.baseurl }}/extension-dev-guide/build/di-xml-file.html#object-lifestyle-configuration) of the class. Default is `true`.
 
+{: .bs-callout .bs-callout-warning}
+The observer name must be unique, or an override will occur.
+
 Below is an example of how to assign observers to watch certain events:
 
 ```xml