Clarify current support level for contracts for materialized views (#…

…5993)

[Preview](https://docs-getdbt-com-git-dbeatty10-patch-1-dbt-labs.vercel.app/docs/collaborate/govern/model-contracts#where-are-contracts-supported)

## What are you changing in this pull request and why?
- It isn't clear if materialized views have support for model contracts
or not.
- Also not clear if custom materializations automatically include
support for model contracts or not.

Context: dbt-labs/dbt-snowflake#888 (`dynamic
table` is the Snowflake equivalent of `materialized view` in other data
platforms)

## Checklist
- [x] I have reviewed 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.

website/docs/docs/collaborate/govern/model-contracts.md

@@ -28,7 +28,8 @@ At present, model contracts are supported for:
 
 Model contracts are _not_ supported for:
 - Python models.
-- `ephemeral`-materialized SQL models.
+- `materialized view` or `ephemeral`-materialized SQL models.
+- Custom materializations (unless added by the author).
 - Models with recursive <Term id="cte" />'s in BigQuery.
 - Other resource types, such as `sources`, `seeds`, `snapshots`, and so on.