Add filters to Advanced Syntax Search docs

doc/user/search/advanced_search_syntax.md

@@ -34,14 +34,33 @@ boolean operators, and much more.
 Full details can be found in the [Elasticsearch documentation][elastic], but
 here's a quick guide:
 
-* Searches look for all the words in a query, in any order - e.g.: searching
+- Searches look for all the words in a query, in any order - e.g.: searching
   issues for `display bug` will return all issues matching both those words, in any order.
-* To find the exact term, use double quotes: `"display bug"`
-* To find bugs not mentioning display, use `-`: `bug -display`
-* To find a bug in display or sound, use `|`: `bug display | sound`
-* To group terms together, use parentheses: `bug | (display +sound)`
-* To match a partial word, use `*`: `bug find_by_*`
-* To find a term containing one of these symbols, use `\`: `argument \-last`
+- To find the exact term, use double quotes: `"display bug"`
+- To find bugs not mentioning display, use `-`: `bug -display`
+- To find a bug in display or sound, use `|`: `bug display | sound`
+- To group terms together, use parentheses: `bug | (display +sound)`
+- To match a partial word, use `*`: `bug find_by_*`
+- To find a term containing one of these symbols, use `\`: `argument \-last`
+
+### Syntax search filters
+
+The Advanced Syntax Search also supports the use of filters. The available filters are:
+
+ - filename: Filters by filename. You can use the glob (`*`) operator for fuzzy matching.
+ - path: Filters by path. You can use the glob (`*`) operator for fuzzy matching.
+ - extension: Filters by extension in the filename. Please write the extension without a leading dot. Exact match only.
+
+To use them, simply add them to your query in the format `<filter_name>:<value>` without
+ any spaces between the colon (`:`) and the value.
+
+Examples:
+
+- Finding a file with any content named `hello_world.rb`: `* filename:hello_world.rb`
+- Finding a file named `hello_world` with the text `whatever` inside of it: `whatever filename:hello_world`
+- Finding the text 'def create' inside files with the `.rb` extension: `def create extension:rb`
+- Finding the text `sha` inside files in a folder called `encryption`: `sha path:encryption`
+- Finding any file starting with `hello` containing `world` and with the `.js` extension: `world filename:hello* extension:js`
 
 [ee]: https://about.gitlab.com/pricing/
 [elastic]: https://www.elastic.co/guide/en/elasticsearch/reference/5.3/query-dsl-simple-query-string-query.html#_simple_query_string_syntax