diesel_derives: specify derive_from_sql_row() docs

[bachmannscode]

This change addresses a terminology detail that may slightly mislead
some library users. The current doc comment states that this derive
implements Queryable for primitive types. "Primitive type" in that
context is supposed to refer to a rust type that corresponds to a
single SQL type in contrast to a whole database row.

However, the current term unfortunately collides with Rust's definition
of "primitive types" [1] so we phrase it in a slightly more elaborate
way to prevent confusion.

[1] https://doc.rust-lang.org/rust-by-example/primitives.html

[weiznich]

Thanks for opening this PR. I agree with you that "primitive type" could refer to the wrong thing there, but I fear that "newtype struct" is even more incorrect as the derive can also be used on different rust types like enums or struct (for representing a sql record type or jsonb type).

[bachmannscode]

Thanks for opening this PR. I agree with you that "primitive type" could refer to the wrong thing there, but I fear that "newtype struct" is even more incorrect as the derive can also be used on different rust types like enums or struct (for representing a sql record type or jsonb type).

Thank you for the review. True, I totally missed that. Do you think one of those could improve the docs:

Implements `Queryable` for types that correspond to a single SQL type. The type has to implement `FromSql`.

or

Implements `Queryable` for types that correspond to a single SQL type (for ones that implement `FromSql`).

?

[weiznich]

I think the first variant sounds better.

[bachmannscode]

I think the first variant sounds better.

Ack, done.

[weiznich]

Thanks 👍

[bachmannscode]

Thank you 💪