add log-format setting to release notes (oops)

This was mistakenly left out of b97b2e858bab¹. oopsie, soz

[1]: b97b2e858b

Change-Id: I1b7210143d66480b6a8a044f0db5f0cd6a6a6964

doc/manual/rl-next/log-format-setting.md

@@ -0,0 +1,20 @@
+---
+synopsis: "Make `log-format` a setting"
+cls: [4686]
+category: "Features"
+credits: [Qyriad]
+---
+
+The [`--log-format` CLI option](@docroot@/command-ref/opt-common.md#opt-log-format) can now be set in [`nix.conf`](@docroot@/command-ref/conf-file.md#conf-log-format)!
+For example, you can now persistently enable the `multiline-with-logs` log format [added in Lix 2.91](@docroot@/release-notes/rl-2.91.md) by adding the following to your `nix.conf`:
+
+```conf
+log-format = multiline-with-logs
+```
+
+Or the equivalent in a NixOS configuration:
+```nix
+{
+  nix.settings.log-format = "multiline-with-logs";
+}
+```