der: Add documentation for the #[asn1(error)] attribute.

der_derive/src/lib.rs

@@ -45,6 +45,21 @@
 //! The default is `EXPLICIT`, so the attribute only needs to be added when
 //! a particular module is declared `IMPLICIT`.
 //!
+//! ### `#[asn1(error = ...)]` attribute: custom error types for decoding
+//!
+//! By default generated `Decode` / `DecodeValue` implementations generated by macros
+//! from this crate use `der::Error` as the generic `Error` parameter, but it's
+//! possible to use a custom error type that implements `From<der::Error>` by using
+//! this attribute.
+//!
+//! Note that [`Choice`] puts more restrictions on the error type: during decoding
+//! for each enum variant the type in its `#[asn1(type = "...")]` attribute (let's
+//! call it `T`) is constructed and then converted to the actual variant's type
+//! (this one will be `U`) using the `TryInto` trait. That means that for each enum
+//! variant's type `U` the custom error type must implement
+//! `From<<U as TryFrom<T>>::Error>`. Since `U` and `T` types are usually the same
+//! implementing `From<Infallible>` should do it.
+//!
 //! ## Field-level attributes
 //!
 //! The following attributes can be added to either the fields of a particular