Refactor advanced search syntax docs

- Rename doc to use underscores
- Mention in search index page

doc/user/search/advanced-search-syntax.md → doc/user/search/advanced_search_syntax.md

-## Advanced search syntax
+# Advanced search syntax
 
-If your site administrator has enabled [Elasticsearch integration](../../integration/elasticsearch.md)
-then some advanced search functionality is available.
+>**Notes:**
+- Introduced in [GitLab Enterprise Premium][ee] 9.2
+- The [Elasticsearch integration](../../integration/elasticsearch.md) needs to
+  be enabled
 
 Full details can be found in the
 [Elasticsearch documentation](https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-simple-query-string-query.html#_simple_query_string_syntax)
@@ -16,3 +18,4 @@ but here's a quick guide:
 * To match a partial word, use `*`: `bug find_by_*`
 * To find a term containing one of these symbols, use `\`: `argument \-last`
 
+[ee]: https://about.gitlab.com/gitlab-ee/

doc/user/search/index.md

@@ -98,3 +98,14 @@ you'll be able to, besides filtering them by **Name**, **Author**, **Assignee**,
 and **Labels**, select multiple issues to add to a list of your choice:
 
 ![search and select issues to add to board](img/search_issues_board.png)
+
+## Advanced search syntax
+
+> Available in [GitLab Enterprise Edition Premium][ee].
+
+If the [Elasticsearch integration](../../integration/elasticsearch.md) is
+enabled, then some advanced search functionality is available.
+
+[Learn how to use the advanced search syntax.](advanced_search_syntax.md)
+
+[ee]: https://about.gitlab.com/gitlab-ee/