Docs: Update query-string-syntax.asciidoc

Closes #6853

docs/reference/query-dsl/queries/query-string-syntax.asciidoc

@@ -23,8 +23,10 @@ search terms, but it is possible to specify other fields in the query syntax:
 
     status:active
 
-* where the `title` field contains `quick` or `brown`
+* where the `title` field contains `quick` or `brown`.
+  If you omit the OR operator the default operator will be used
 
+    title:(quick OR brown)
     title:(quick brown)
 
 * where the `author` field contains the exact phrase `"john smith"`