[ticket/12286] Add "Since" to template event docs

PHPBB3-12286

phpBB/docs/coding-guidelines.html

@@ -1707,6 +1707,7 @@ parent = prosilver
 * Location: styles/<style_name>/template/filename.html
 * Purpose: A brief description of what this event should be used for.
 This may span multiple lines.
+* Since: Version since when the event was added
 </pre></div></li>
 		<li>An event found in multiple template files:
 		<div class="codebox"><pre>event_name
@@ -1715,6 +1716,7 @@ This may span multiple lines.
     + first/file/path.html
     + second/file/path.html
 * Purpose: Same as above.
+* Since: 3.1.0-b1
 </pre></div>
 		<li>An event that is found multiple times in a file should have the number of instances in parenthesis next to the filename.
 		<div class="codebox"><pre>event_name
@@ -1723,6 +1725,7 @@ This may span multiple lines.
     + first/file/path.html (2)
     + second/file/path.html
 * Purpose: Same as above.
+* Since: 3.1.0-b1
 </pre></div></li>
 		<li>An actual example event documentation:
 		<div class="codebox"><pre>forumlist_body_last_post_title_prepend
@@ -1730,7 +1733,9 @@ This may span multiple lines.
 * Locations:
     + styles/prosilver/template/forumlist_body.html
     + styles/subsilver2/template/forumlist_body.html
-* Purpose: Add content before the post title of the latest post in a forum on the forum list.</pre></div></ul><br />
+* Purpose: Add content before the post title of the latest post in a forum on the forum list.
+* Since: 3.1.0-a1
+</pre></div></ul><br />
 
 		</div>