Update custom-schemas.md

The [generate_schema_name_for_env](https://github.com/dbt-labs/dbt-adapters/blob/main/dbt-adapters/src/dbt/include/global_project/macros/get_custom_name/get_custom_schema.sql#L47-L60) macro uses the target.schema as a fallback when no custom schema is set. Updating the wording here to be a little more precise regarding how the marco functions.

website/docs/docs/build/custom-schemas.md

@@ -161,7 +161,7 @@ See docs on macro `dispatch`: ["Managing different global overrides across packa
 
 ## A built-in alternative pattern for generating schema names
 
-A common customization is to ignore the target schema in production environments, and ignore the custom schema configurations in other environments (such as development and CI).
+A common customization is to use the custom schema in production when set, with the target schema serving only as a fallback if no custom schema is specified. In other environments, such as development and CI, custom schema configurations are ignored.
 
 Production Environment (`target.name == 'prod'`)