Document that `serial` and `generated` columns are not in `insert` and `update` mutations

[olirice]

Document that setting a type as serial or generated removes those fields from the insert and update types so end users can't provide a value and they're always generated by postgres

This is intentional behavior so users can't, for example, select their own value for a table's id, which will later by emitted by sequence which will conflict and create raise an error.

[olirice]

ref https://github.com/supabase/pg_graphql/issues/470#issuecomment-2005729498