Small cleanup for ECS error handling (#18280)

# Objective

While poking at https://github.com/bevyengine/bevy/issues/17272, I
noticed a few small things to clean up.

## Solution

- Improve the docs
- ~~move `SystemErrorContext` out of the `handler.rs` module: it's not
an error handler~~

crates/bevy_app/src/app.rs

@@ -1276,7 +1276,7 @@ impl App {
 
     /// Set the global system error handler to use for systems that return a [`Result`].
     ///
-    /// See the [`bevy_ecs::result` module-level documentation](../../bevy_ecs/result/index.html)
+    /// See the [`bevy_ecs::error` module-level documentation](bevy_ecs::error)
     /// for more information.
     pub fn set_system_error_handler(
         &mut self,

crates/bevy_app/src/sub_app.rs

@@ -338,7 +338,7 @@ impl SubApp {
 
     /// Set the global error handler to use for systems that return a [`Result`].
     ///
-    /// See the [`bevy_ecs::result` module-level documentation](../../bevy_ecs/result/index.html)
+    /// See the [`bevy_ecs::error` module-level documentation](bevy_ecs::error)
     /// for more information.
     pub fn set_system_error_handler(
         &mut self,

crates/bevy_ecs/src/error/mod.rs

@@ -8,10 +8,10 @@
 //! [`panic`] error handler function is used, resulting in a panic with the error message attached.
 //!
 //! You can change the default behavior by registering a custom error handler, either globally or
-//! per `Schedule`:
+//! per [`Schedule`]:
 //!
-//! - [`App::set_system_error_handler`] sets the global error handler for all systems of the
-//!   current [`World`].
+//! - `App::set_system_error_handler` (via `bevy_app`) sets the global error handler for all systems of the
+//!   current [`World`] by modifying the [`DefaultSystemErrorHandler`].
 //! - [`Schedule::set_error_handler`] sets the error handler for all systems of that schedule.
 //!
 //! Bevy provides a number of pre-built error-handlers for you to use:
@@ -76,5 +76,7 @@ mod handler;
 pub use bevy_error::*;
 pub use handler::*;
 
-/// A result type for use in fallible systems.
+/// A result type for use in fallible systems, commands and observers.
+///
+/// The [`BevyError`] type is a type-erased error type with optional Bevy-specific diagnostics.
 pub type Result<T = (), E = BevyError> = core::result::Result<T, E>;