Write introduction for #[\Override] attribute (#4075)

Co-authored-by: Tim Düsterhus <[email protected]>

Author: mmalferov

language/predefined/attributes/override.xml

@@ -7,8 +7,16 @@
 
   <section xml:id="override.intro">
    &reftitle.intro;
-   <para>
-   </para>
+   <simpara>
+    This attribute is used to indicate that a method is intended
+    to override a method of a parent class or that it implements
+    a method defined in an interface.
+   </simpara>
+   <simpara>
+    If no method with the same name exists in a parent class or
+    in an implemented interface a compile-time error will be
+    emitted.
+   </simpara>
   </section>
 
   <section xml:id="override.synopsis">