add documentation for unit testing incremental models (#4959)

## What are you changing in this pull request and why?
<!---
Describe your changes and why you're making them. If related to an open 
issue or a pull request on dbt Core, then link to them here! 

To learn more about the writing conventions used in the dbt Labs docs,
see the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md).
-->

Closes #4929

## Checklist
<!--
Uncomment when publishing docs for a prerelease version of dbt:
- [ ] Add versioning components, as described in [Versioning
Docs](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-entire-pages)
- [ ] Add a note to the prerelease version [Migration
Guide](https://github.com/dbt-labs/docs.getdbt.com/tree/current/website/docs/docs/dbt-versions/core-upgrade)
-->
- [ ] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.
- [ ] For [docs
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#about-versioning),
review how to [version a whole
page](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
and [version a block of
content](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-blocks-of-content).
- [ ] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

Adding or removing pages (delete if not applicable):
- [ ] Add/remove page in `website/sidebars.js`
- [ ] Provide a unique filename for new pages
- [ ] Add an entry for deleted pages in `website/static/_redirects`
- [ ] Run link testing locally with `npm run build` to update the links
that point to deleted pages

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

@@ -255,5 +255,74 @@ unit_tests:
 
 ```
 
+## Unit testing incremental models
 
+When configuring your unit test, you can override the output of macros, vars, or environment variables. This enables you to unit test your incremental models in "full refresh" and "incremental" modes. 
 
+When testing an incremental model, the expected output is the __result of the materialization__ (what will be merged/inserted), not the resulting model itself (what the final table will look like after the merge/insert).
+
+For example, say you have an incremental model in your 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 %}
+
+```
+
+</File>
+
+You can define unit tests on `my_incremental_model` to ensure your 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 inserted/merged the records into your existing model correctly, but [we're investigating support for 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