[Docs] Mention reserved completion suggestion characters (#48445)

We currently don't mention the three reserved characters anywhere. This change
adds a short note mentioning them

Closes #48341

Author: Christoph Büscher

docs/reference/search/suggesters/completion-suggest.asciidoc

@@ -100,6 +100,13 @@ The following parameters are supported:
 `input`::
     The input to store, this can be an array of strings or just
     a string. This field is mandatory.
++
+NOTE: This value cannot contain the following UTF-16 control characters:
+
+* `\u0000` (null)
+* `\u001f` (information separator one)
+* `\u001e` (information separator two)
+
 
 `weight`::
     A positive integer or a string containing a positive integer,