Updating freshness ref page (#4770)

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

Updating the `freshness` ref page to include new information for v1.7

Closes #4615

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website/docs/reference/resource-properties/freshness.md

@@ -37,6 +37,38 @@ A freshness block is used to define the acceptable amount of time between the mo
 
 In the `freshness` block, one or both of `warn_after` and `error_after` can be provided. If neither is provided, then dbt will not calculate freshness snapshots for the tables in this source.
 
+<VersionBlock firstVersion="1.7">
+
+In most cases, the `loaded_at_field` is required. Some adapters support calculating source freshness from the warehouse metadata tables and can exclude the `loaded_at_field`.
+
+If a source has a `freshness:` block, dbt will attempt to calculate freshness for that source:
+- If a `loaded_at_field` is provided, dbt will calculate freshness via a select query (behavior prior to v1.7).
+- If a `loaded_at_field` is _not_ provided, dbt will calculate freshness via warehouse metadata tables when possible (new in v1.7 on supported adapters).
+
+Currently, calculating freshness from warehouse metadata tables is supported on:
+- [Snowflake](/reference/resource-configs/snowflake-configs)
+
+Support is coming soon to the following adapters:
+- [Redshift](/reference/resource-configs/redshift-configs)
+- [BigQuery](/reference/resource-configs/bigquery-configs)
+- [Spark](/reference/resource-configs/spark-configs)
+
+Freshness blocks are applied hierarchically:
+- a `freshness` and `loaded_at_field` property added to a source will be applied to all all tables defined in that source
+- a `freshness` and `loaded_at_field` property added to a source _table_ will override any properties applied to the source.
+
+This is useful when all of the tables in a source have the same `loaded_at_field`, as is often the case.
+
+To exclude a source from freshness calculations, you have two options:
+- Don't add a `freshness:` block.
+- Explicitly set `freshness: null`.
+
+## loaded_at_field
+(Optional on adapters that support pulling freshness from warehouse metadata tables, required otherwise.)
+</VersionBlock>
+
+<VersionBlock lastVersion="1.6">
+
 Additionally, the `loaded_at_field` is required to calculate freshness for a table. If a `loaded_at_field` is not provided, then dbt will not calculate freshness for the table.
 
 Freshness blocks are applied hierarchically:
@@ -47,7 +79,7 @@ This is useful when all of the tables in a source have the same `loaded_at_field
 
 ## loaded_at_field
 (Required)
-
+</VersionBlock>
 A column name (or expression) that returns a timestamp indicating freshness.
 
 If using a date field, you may have to cast it to a timestamp: