Clarify relationship between general SQL types and backend-specific types

[jimmycuadra]

The backend-specific types, e.g. diesel::data_types, contain some types that have seemingly identical counterparts in diesel::types. For example, there is both diesel::types::Timestamp and diesel::data_types::PgTimestamp. It's not clear why you would use one over the other. The docs for the backend-specific types say they are for cases where there is no native Rust type, and yet matching types appear in the generic types module anyway. Are both of them user-facing types? Is one an implementation detail that happens to be public? If you're using Postgres and the table! macro, which one do you use to declare a timestamp column? The docs should explain this. The module names types and data_types also have no semantic distinction which adds to the confusion.

[sgrif]

You would always use types::Timestamp in the table macro. data_types::PgTimestamp is something that can be deserialized from a column of that type.

[jimmycuadra]

Thanks! I can make a PR to add some docs explaining this. Is there anything else actionable here? Maybe a way to name the types and data_types module that's more clear?