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 #470 (comment)