docs(topic): Point out value of custom context

src/_topic/error.rs

@@ -1,25 +1,40 @@
 //! # Custom Errors
 //!
-//! Between [`ContextError`], [`Parser::context`], and [`cut_err`],
-//! most error needs will likely be met
-//! (see [tutorial][chapter_7]).
-//! When that isn't the case, you can implement your own error type.
+//! A lot can be accomplished with the built-in error tools, like:
+//! - [`ContextError`]
+//! - [`Parser::context`]
+//! - [`cut_err`]
 //!
-//! The most basic error trait is [`ParserError`].
+//! *(see [tutorial][chapter_7])*
+//!
+//! Most other needs can likely be met by using a custom context type with [`ContextError`] instead
+//! of [`StrContext`].
+//! This will require implementing a custom renderer.
+//!
+//! ## `ParserError` Trait
+//!
+//! When needed, you can also create your own type that implements [`ParserError`].
 //!
 //! Optional traits include:
 //! - [`AddContext`]
 //! - [`FromExternalError`]
 //!
-//! # Example
+//! There are multiple strategies for implementing support for [`AddContext`] and [`FromExternalError`]:
+//! - Make your error type generic over the context or external error
+//! - Require a trait for the context or external error and `Box` it
+//! - Make the context an enum like [`StrContext`]
+//! - Implement the trait multiple times, one for each concrete context or external error type,
+//!   allowing custom behavior per type
 //!
+//! Example:
 //!```rust
 #![doc = include_str!("../../examples/custom_error.rs")]
 //!```
 
 #![allow(unused_imports)]
 use crate::combinator::cut_err;
 use crate::error::ContextError;
+use crate::error::StrContext;
 use crate::Parser;
 use crate::_tutorial::chapter_7;
 use crate::error::AddContext;