diesel_derives: specify derive_from_sql_row() docs

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

diesel_derives/src/lib.rs

@@ -218,7 +218,7 @@ pub fn derive_diesel_numeric_ops(input: TokenStream) -> TokenStream {
     diesel_numeric_ops::derive(parse_macro_input!(input)).into()
 }
 
-/// Implements `Queryable` for primitive types
+/// Implements `Queryable` for types that correspond to a single SQL type. The type must implement `FromSql`.
 ///
 /// This derive is mostly useful to implement support deserializing
 /// into rust types not supported by Diesel itself.