add documentation for unit testing incremental models

website/docs/docs/build/unit-tests.md

@@ -254,5 +254,72 @@ unit_tests:
 
 ```
 
+## Unit testing incremental models
 
+When configuring your unit test, you can override the output of macros, vars, or environment variables for a given unit test. This enables you to unit test your incremental models - both in "full refresh" and "incremental" mode. 
 
+When testing an incremental model, the expected output is the __result of the materialization__ (i.e. what will be merged/inserted), not the resulting model itself (i.e. what the final table will look like after the merge/insert).
+
+Let's say I have an incremental model in my project:
+
+<file name='my_incremental_model.sql'>
+
+```sql
+
+{{
+    config(
+        materialized='incremental'
+    )
+}}
+
+select * from {{ ref('events') }}
+{% if is_incremental() %}
+where event_time > (select max(event_time) from {{ this }})
+{% endif %}
+
+```
+
+I can define unit tests on `my_incremental_model` to ensure my incremental logic is working as expected:
+
+```yml
+
+unit_tests:
+  - name: my_incremental_model_full_refresh_mode
+    model: my_incremental_model
+    overrides:
+      macros:
+        # unit test this model in "full refresh" mode
+        is_incremental: false 
+    given:
+      - input: ref('events')
+        rows:
+          - {event_id: 1, event_time: 2020-01-01}
+    expect:
+      rows:
+        - {event_id: 1, event_time: 2020-01-01}
+
+  - name: my_incremental_model_incremental_mode
+    model: my_incremental_model
+    overrides:
+      macros:
+        # unit test this model in "incremental" mode
+        is_incremental: true 
+    given:
+      - input: ref('events')
+        rows:
+          - {event_id: 1, event_time: 2020-01-01}
+          - {event_id: 2, event_time: 2020-01-02}
+          - {event_id: 3, event_time: 2020-01-03}
+      - input: this 
+        # contents of current my_incremental_model
+        rows:
+          - {event_id: 1, event_time: 2020-01-01}
+    expect:
+      # what will be inserted/merged into my_incremental_model
+      rows:
+        - {event_id: 2, event_time: 2020-01-02}
+        - {event_id: 3, event_time: 2020-01-03}
+
+```
+
+There is currently no way to unit test whether the dbt framework actually inserted/merged the records into your existing model correctly or not - but [we're investigating a way to support this in the future](https://github.com/dbt-labs/dbt-core/issues/8664).

website/docs/reference/resource-properties/unit-tests.md

@@ -153,6 +153,10 @@ When `format: csv`, can either supply:
     - If you do supply an input for a seed, we will use that input instead.
 - You can also have “empty” inputs, by setting rows to an empty list `rows: []`
 
+### Overrides
+
+When configuring your unit test, you can override the output of macros, vars, or environment variables for a given unit test. 
+
 ## Examples
 ```yml