adds event documentation

xtask/res/templates/runtime-events.md

@@ -15,6 +15,7 @@ There are **{{event_counter}}** events from **{{ pallets | length }}** pallets.
 {{ param.name }}{% if loop.last != true %}, {% endif %} 
 {%- endfor -%}
 )</code> - {{ event.index }}</summary>
+{{ event.documentation }}
 
 ```rust
 {% for param in event.params -%}