Better explain the message changes

changelog.d/pr-10546

@@ -1,9 +1,74 @@
 ---
-synopsis: Add an Ord instance for ProjectConfigPath
+synopsis: Deduplicate "using configuration from" message
 packages: [cabal-install-solver]
 prs: 10546
 ---
 
-Add an `Ord` instance for `ProjectConfigPath` that sorts URIs after local paths
-and longer paths after shorter ones. Deduplicate the printing of "Configuration is
-affected by the following files" messages.
+## Using Configuration From Message Changes
+
+Deduplicates and sorts the list of configuration files and URIs printed with the
+"using configuration from" messages. This is the message shown when there's a
+build failure. We can trigger that message by using a non-existant package in
+the project, "no-pkg-dir".
+
+If an import is repeated in a `.project` or `.config` file it only imported once
+but if the same import is made from an imported file then it is repeated in the
+message.
+
+* The test set up:
+
+    ```
+    $ cat cabal.project
+    packages: no-pkg-dir
+    import: extra.config
+    import: extra.config
+    import: very-extra.config
+    import: very-extra.config
+    import: https://www.stackage.org/lts-21.25/cabal.config
+    import: https://www.stackage.org/lts-21.25/cabal.config
+
+    $ cat extra.config
+    import: https://www.stackage.org/lts-21.25/cabal.config
+    import: https://www.stackage.org/lts-21.25/cabal.config
+
+    $ cat very-extra.config
+    import: https://www.stackage.org/lts-21.25/cabal.config
+    import: https://www.stackage.org/lts-21.25/cabal.config
+    ```
+
+* Before the fix:
+
+    ```
+    $ ~/.ghcup/bin/cabal-3.12.1.0 build all --dry-run
+    When using configuration from:
+    - cabal.project
+    - extra.config
+    - https://www.stackage.org/lts-21.25/cabal.config
+    - https://www.stackage.org/lts-21.25/cabal.config
+    - https://www.stackage.org/lts-21.25/cabal.config
+    - very-extra.config
+    The following errors occurred:
+    - The package location 'no-pkg-dir' does not exist.
+    ```
+
+* After the fix:
+
+    ```
+    $ cabal build all --dry-run
+    When using configuration from:
+    - cabal.project
+    - extra.config
+    - very-extra.config
+    - https://www.stackage.org/lts-21.25/cabal.config
+    The following errors occurred:
+    - The package location 'no-pkg-dir' does not exist.
+    ```
+
+## Ord ProjectConfigPath Instance Changes
+
+Adds a custom `Ord` instance for `ProjectConfigPath` that sorts URIs after local
+file paths and longer file paths after shorter ones as measured by the number of
+path segments. If still equal, then sorting is lexical.  The project itself, a
+single element root path, compared to any of the configuration paths it imports,
+should always sort first. Comparing one project root path against another is
+done lexically.