chore: add some comment for make_request_to_vault function (#8420)

Co-authored-by: soulbird <zhaothree@gmail.com>

apisix/kms/vault.lua

@@ -45,7 +45,12 @@ local _M = {
     schema = schema
 }
 
-
+-- This code is copied from apisix/core/vault.lua.
+-- The functions in apisix/core/vault.lua are currently only used in the jwt-auth plugin,
+-- and it is not suitable to be placed in the core module of APISIX.
+--
+-- When KMS is fully functional, we will remove apisix/core/vault.lua.
+--
 local function make_request_to_vault(conf, method, key, data)
     local httpc = http.new()
     -- config timeout or default to 5000 ms