Expand on 1.85.1 merged doctests

Author: cuviper

posts/Rust-1.85.1.md

@@ -24,7 +24,16 @@ appropriate page on our website.
 
 ## What's in 1.85.1
 
-- [Fix the doctest-merging feature of the 2024 Edition.](https://github.com/rust-lang/rust/pull/137899/)
+### Fixed 2024 Edition doctests
+
+Due to a bug in the implementation, [combined doctests](https://doc.rust-lang.org/edition-guide/rust-2024/rustdoc-doctests.html) did not work as intended in the stable 2024 Edition. Internal errors with feature stability caused rustdoc to automatically use its "unmerged" fallback method instead, like previous editions.
+
+Those errors are now fixed in 1.85.1, realizing the performance improvement of combined doctest compilation as intended! See the [backport issue](https://github.com/rust-lang/rust/issues/138418) for more details, including the risk analysis of making this behavioral change in a point release.
+
+### Other fixes
+
+1.85.1 also resolves a few regressions introduced in 1.85.0:
+
 - [Relax some `target_feature` checks when generating docs.](https://github.com/rust-lang/rust/pull/137632/)
 - [Fix errors in `std::fs::rename` on Windows 1607.](https://github.com/rust-lang/rust/pull/137528/)
 - [Downgrade bootstrap `cc` to fix custom targets.](https://github.com/rust-lang/rust/pull/137460/)