chore(breaking-changes): add details to no_std-related breaking changes (#1828)

Some corrections and added details to BREAKING-CHANGES.md.

I decided to remove:

- `Backend` now uses `Self::Error` for error handling instead of
`std::io::Error`
- `Terminal<B>` now uses `B::Error` for error handling instead of
`std::io::Error`

...as we are still using `std::io::Error` in built-in backends, so this
will only be breaking if a third-party backend decides to use a custom
error other than `std::io::Error`, which would be a breaking change in
downstream and not `ratatui`.

The exception to that is `TestBackend`, which uses `Infallible`, but
this already has its own breaking changes entry.

BREAKING-CHANGES.md

@@ -17,8 +17,6 @@ This is a quick summary of the sections below:
   - 'layout::Alignment' is renamed to 'layout::HorizontalAlignment'
   - The MSRV is now 1.81.0
   - `Backend` now requires an associated `Error` type and `clear_region` method
-  - `Backend` now uses `Self::Error` for error handling instead of `std::io::Error`
-  - `Terminal<B>` now uses `B::Error` for error handling instead of `std::io::Error`
   - `TestBackend` now uses `core::convert::Infallible` for error handling instead of `std::io::Error`
   - Disabling `default-features` will now disable layout cache, which can have a negative impact on performance
   - `Layout::init_cache` and `Layout::DEFAULT_CACHE_SIZE` are now only available if `layout-cache` feature is enabled
@@ -121,6 +119,16 @@ re-enable layout cache. Not doing so may impact performance.
 Since `TestBackend` never fails, it now uses `Infallible` as associated `Error`. This may require
 changes in test cases that use `TestBackend`.
 
+### `Backend` now requires an associated `Error` type and `clear_region` method ([#1778])
+
+[#1778]: https://github.com/ratatui/ratatui/pull/1778
+
+Custom `Backend` implementations must now define an associated `Error` type for method `Result`s
+and implement the `clear_region` method, which no longer has a default implementation.
+
+This change was made to provide greater flexibility for custom backends, particularly to remove the
+explicit dependency on `std::io` for backends that want to support `no_std` targets.
+
 ### The MSRV is now 1.81.0 ([#1786])
 
 [#1786]: https://github.com/ratatui/ratatui/pull/1786