update pk docs for sql databases to avoid confusion

docs/en/docs/tutorial/sql-databases.md

@@ -67,6 +67,10 @@ There are a few differences:
 
     By having the type as `int | None`, SQLModel will know that this column should be an `INTEGER` in the SQL database and that it should be `NULLABLE`.
 
+    But inside the SQL database, the `id` is always required and can't be `NULL`. So, why should we declare it with `int | None`?
+
+    The `id` will be required in the database, but it will be generated by the database, not by our code.
+
 * `Field(index=True)` tells SQLModel that it should create a **SQL index** for this column, that would allow faster lookups in the database when reading data filtered by this column.
 
     SQLModel will know that something declared as `str` will be a SQL column of type `TEXT` (or `VARCHAR`, depending on the database).