Add `full` output format changes to the changelog (#19968)

Summary
--

I thought this might warrant a small blog-style writeup, especially
since we already got a question about it (#19966), but I'm happy to
switch back to a one-liner under `### Other changes` if preferred.

I'll copy whatever we add here to the release notes too.

Do we need a note at the top about the late addition?

CHANGELOG.md

@@ -24,8 +24,31 @@
 ### Other changes
 
 - Build `riscv64` binaries for release ([#19819](https://github.com/astral-sh/ruff/pull/19819))
+
 - Add rule code to error description in GitLab output ([#19896](https://github.com/astral-sh/ruff/pull/19896))
 
+- Improve rendering of the `full` output format ([#19415](https://github.com/astral-sh/ruff/pull/19415))
+
+    Below is an example diff for [`F401`](https://docs.astral.sh/ruff/rules/unused-import/):
+
+    ```diff
+    -unused.py:8:19: F401 [*] `pathlib` imported but unused
+    +F401 [*] `pathlib` imported but unused
+    +  --> unused.py:8:19
+        |
+      7 | # Unused, _not_ marked as required (due to the alias).
+      8 | import pathlib as non_alias
+    -   |                   ^^^^^^^^^ F401
+    +   |                   ^^^^^^^^^
+      9 |
+     10 | # Unused, marked as required.
+        |
+    -   = help: Remove unused import: `pathlib`
+    +help: Remove unused import: `pathlib`
+    ```
+
+    For now, the primary difference is the movement of the filename, line number, and column information to a second line in the header. This new representation will allow us to make further additions to Ruff's diagnostics, such as adding sub-diagnostics and multiple annotations to the same snippet.
+
 ## 0.12.8
 
 ### Preview features