docs: Add documentation for aider/default and aider/override model settings

aider/website/docs/config/adv-model-settings.md

@@ -55,8 +55,28 @@ These model settings are pre-configured for most popular models.
 But it can sometimes be helpful to override them or add settings for
 a model that aider doesn't know about.
 
-To do that,
-create a `.aider.model.settings.yml` file in one of these locations:
+### Default and override settings
+
+You can define default settings that apply to all models, and override settings that are applied last to all models.
+To do this, add special model entries named `aider/default` and `aider/override` to your model settings file.
+
+For example, to make all models use the "diff" edit format by default, but force the "whole" format for specific models:
+
+```yaml
+- name: aider/default
+  edit_format: diff
+  use_repo_map: true
+
+- name: some-specific-model
+  edit_format: whole  # Overrides the default
+
+- name: aider/override
+  use_temperature: false  # Applied last to all models
+```
+
+### Configuration file locations
+
+Create a `.aider.model.settings.yml` file in one of these locations:
 
 - Your home directory.
 - The root if your git repo.