documentation fix: index exists is ambiguous (#28460)

docs/reference/indices/indices-exists.asciidoc

@@ -12,3 +12,6 @@ HEAD twitter
 
 The HTTP status code indicates if the index exists or not. A `404` means
 it does not exist, and `200` means it does.
+
+IMPORTANT: This request does not distinguish between an index and an alias,
+i.e. status code `200` is also returned if an alias exists with that name.