Update rl-3.5.1.md

Author: grahamc

doc/manual/source/release-notes-determinate/rl-3.5.1.md

@@ -3,10 +3,55 @@
 * Based on [upstream Nix 2.28.3](../release-notes/rl-2.28.md).
 
 ## What's Changed
-* Deprecate upgrade-nix command in favor of `determinate-nixd upgrade`, by @gustavderdrache in [DeterminateSystems/nix-src#55](https://github.com/DeterminateSystems/nix-src/pull/55)
+
+Most notably, Lazy Trees has merged in to Determinate Nix and is in Feature Preview status, but remains disabled by default.
+Lazy trees massively improves performance in virtually all scenarios because it enables Nix to avoid making unnecessary copies of files into the Nix store.
+In testing, we saw iteration times on Nixpkgs **drop from over 12 seconds to 3.5 seconds**.
+
+After upgrading to Determinate Nix 3.5.1 with `sudo determinate-nixd upgrade`, enable lazy trees by adding this to `/etc/nix/nix.custom.conf`:
+
+```
+lazy-trees = true
+```
+
+Please note that our full flake regression test suite passes with no changes with lazy trees, and please report compatibility issues.
+
+Read [this GitHub comment](https://github.com/DeterminateSystems/nix-src/pull/27#pullrequestreview-2822153088) for further details and next steps.
+We'll be publishing an update on the [Determinate Systems blog](https://determinate.systems/posts/) in the next few days with more information as well.
+
+Relevant PRs:
 * Lazy trees v2 by @edolstra in [DeterminateSystems/nix-src#27](https://github.com/DeterminateSystems/nix-src/pull/27)
 * Improve lazy trees backward compatibility by @edolstra in [DeterminateSystems/nix-src#56](https://github.com/DeterminateSystems/nix-src/pull/56)
-* Canonicalize flake input URLs before checking flake.lock file staleness, for dealing with `dir` in URL-style flakerefs by @edolstra in [DeterminateSystems/nix-src#57](https://github.com/DeterminateSystems/nix-src/pull/57)
-* Improve build failure error messages by @edolstra in [DeterminateSystems/nix-src#58](https://github.com/DeterminateSystems/nix-src/pull/58)
+
+
+### Additional changes in this release:
+* Bug fix: Flake input URLs are canonicalized before checking flake.lock file staleness, avoiding needlessly regenerating flake.lock files with `dir` in URL-style flakerefs by @edolstra in [DeterminateSystems/nix-src#57](https://github.com/DeterminateSystems/nix-src/pull/57)
+* `nix upgrade-nix` is deprecated in favor of `determinate-nixd upgrade`, by @gustavderdrache in [DeterminateSystems/nix-src#55](https://github.com/DeterminateSystems/nix-src/pull/55)
+* UX: Improved build failure and dependency failure error messages to include needed output paths by @edolstra in [DeterminateSystems/nix-src#58](https://github.com/DeterminateSystems/nix-src/pull/58).
+
+Previously:
+
+```
+error: builder for '/nix/store/[...]-nested-failure-bottom.drv' failed with exit code 1
+error: 1 dependencies of derivation '/nix/store/[...]-nested-failure-middle.drv' failed to build
+error: 1 dependencies of derivation '/nix/store/[...]-nested-failure-top.drv' failed to build
+```
+
+Now:
+
+```
+error: Cannot build '/nix/store/w37gflm9wz9dcnsgy3sfrmnlvm8qigaj-nested-failure-bottom.drv'.
+       Reason: builder failed with exit code 1.
+       Output paths:
+         /nix/store/yzybs8kp35dfipbzdlqcc6lxz62hax04-nested-failure-bottom
+error: Cannot build '/nix/store/00gr5hlxfc03x2675w6nn3pwfrz2fr62-nested-failure-middle.drv'.
+       Reason: 1 dependency failed.
+       Output paths:
+         /nix/store/h781j5h4bdchmb4c2lvy8qzh8733azhz-nested-failure-middle
+error: Cannot build '/nix/store/8am0ng1gyx8sbzyr0yx6jd5ix3yy5szc-nested-failure-top.drv'.
+       Reason: 1 dependency failed.
+       Output paths:
+         /nix/store/fh12637kgvp906s9yhi9w2dc7ghfwxs1-nested-failure-top
+```
 
 **Full Changelog**: [v3.4.2...v3.5.1](https://github.com/DeterminateSystems/nix-src/compare/v3.4.2...v3.5.1)