diff --git a/docs/reference/query-dsl/query-string-syntax.asciidoc b/docs/reference/query-dsl/query-string-syntax.asciidoc
index 8a76d686b62..8a7b394b2e8 100644
--- a/docs/reference/query-dsl/query-string-syntax.asciidoc
+++ b/docs/reference/query-dsl/query-string-syntax.asciidoc
@@ -53,6 +53,25 @@ Be aware that wildcard queries can use an enormous amount of memory and
 perform very badly -- just think how many terms need to be queried to
 match the query string `"a* b* c*"`.
 
+[WARNING]
+=======
+Pure wildcards `\*` are rewritten to <<query-dsl-exists-query,`exists`>> queries for efficiency.
+As a consequence, the wildcard `"field:*"` would match documents with an emtpy value
+ like the following:
+```
+{
+  "field": ""
+}
+```
+\... and would **not** match if the field is missing or set with an explicit null
+value like the following:
+```
+{
+  "field": null
+}
+```
+=======
+
 [WARNING]
 =======
 Allowing a wildcard at the beginning of a word (eg `"*ing"`) is particularly