Add note specifying unsupported @ operator for mysql (sqlc-dev#3190)

Author: jakoguta

docs/howto/named_parameters.md

@@ -46,6 +46,10 @@ type UpdateAuthorNameParams struct {
 If the `sqlc.arg()` syntax is too verbose for your taste, you can use the `@`
 operator as a shortcut.
 
+```{note}
+The `@` operator as a shortcut for `sqlc.arg()` is not supported in MySQL.
+```
+
 ```sql
 -- name: UpsertAuthorName :one
 UPDATE author