Documenting custom materializations (#5211)

[Preview](https://docs-getdbt-com-git-dbeatty10-patch-1-dbt-labs.vercel.app/faqs/Docs/documenting-macros#document-a-custom-materialization)

## What are you changing in this pull request and why?

dbt-labs/dbt-core#9841 asked about documenting
custom materializations, and this PR explains how to document the
macro(s) associated with each custom materialization.

## Checklist
- [x] 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.
- [x] I tested that the code example works

website/docs/faqs/Docs/documenting-macros.md

@@ -27,3 +27,26 @@ macros:
 ```
 
 </File>
+
+## Document a custom materialization
+
+When you create a [custom materialization](/guides/create-new-materializations), dbt creates an associated macro with the following format:
+```
+materialization_{materialization_name}_{adapter}
+```
+
+To document a custom materialization, use the previously mentioned format to determine the associated macro name(s) to document.
+
+<File name='macros/properties.yml'>
+
+```yaml
+version: 2
+
+macros:
+  - name: materialization_my_materialization_name_default
+    description: A custom materialization to insert records into an append-only table and track when they were added.
+  - name: materialization_my_materialization_name_xyz
+    description: A custom materialization to insert records into an append-only table and track when they were added.
+```
+
+</File>