docs: correct on_key docs (#27429)

runtime/lua/vim/_editor.lua

@@ -652,8 +652,9 @@ local on_key_cbs = {}
 ---@note {fn} will not be cleared by |nvim_buf_clear_namespace()|
 ---@note {fn} will receive the keys after mappings have been evaluated
 ---
----@param fn fun(key: string) Function invoked on every key press. |i_CTRL-V|
----                   Returning nil removes the callback associated with namespace {ns_id}.
+---@param fn fun(key: string)? Function invoked on every key press. |i_CTRL-V|
+---                   Passing in nil when {ns_id} is specified removes the
+---                   callback associated with namespace {ns_id}.
 ---@param ns_id integer? Namespace ID. If nil or 0, generates and returns a
 ---                     new |nvim_create_namespace()| id.
 ---