Adding new state selectors (#3596)

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

Adding the `state:unmodified` and `state:old` selectors

## Checklist

- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
and [About
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
so my content adheres to these guidelines.
- [x] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

website/docs/reference/node-selection/methods.md

@@ -208,6 +208,16 @@ Because state comparison is complex, and everyone's project is different, dbt su
 
 Remember that `state:modified` includes _all_ of the criteria above, as well as some extra resource-specific criteria, such as modifying a source's `freshness` or `quoting` rules or an exposure's `maturity` property. (View the source code for the full set of checks used when comparing [sources](https://github.com/dbt-labs/dbt-core/blob/9e796671dd55d4781284d36c035d1db19641cd80/core/dbt/contracts/graph/parsed.py#L660-L681), [exposures](https://github.com/dbt-labs/dbt-core/blob/9e796671dd55d4781284d36c035d1db19641cd80/core/dbt/contracts/graph/parsed.py#L768-L783), and [executable nodes](https://github.com/dbt-labs/dbt-core/blob/9e796671dd55d4781284d36c035d1db19641cd80/core/dbt/contracts/graph/parsed.py#L319-L330).)
 
+<VersionBlock firstVersion="1.6">
+
+There are two additional `state` selectors that complement `state:new` and `state:modified` by representing the inverse of those functions:
+- `state:old` &mdash; A node with the same `unique_id` exists in the comparison manifest
+- `state:unmodified` &mdash; All existing nodes with no changes 
+
+These selectors can help you shorten run times by excluding unchanged nodes. Currently, no subselectors are available at this time, but that might change as use cases evolve. 
+
+</VersionBlock>
+
 ### The "exposure" method
 
 The `exposure` method is used to select parent resources of a specified [exposure](/docs/build/exposures). Use in conjunction with the `+` operator.