Added clarifying line for geography retire_date

Author: schnuerle

geography/README.md

@@ -95,7 +95,7 @@ See the [Endpoints](#endpoints) below for links to their specific JSON Schema do
 | `geography_json`   | JSON      | Required   | The GeoJSON that defines the geographical coordinates.                                 |
 | `effective_date`   | [timestamp][ts] | Optional   | The date at which a Geography is considered "live".  Must be at or after `published_date`. |
 | `published_date`     | [timestamp][ts] | Required   | Time that the geography was published, i.e. made immutable                       |
-| `retire_date`     | [timestamp][ts] | Optional   | Time that the geography is slated to retire. Once the retire date is passed, new policies can no longer reference it and old policies referencing it should be updated. Retired geographies should continue to be returned in the geographies list. Must be after `effective_date`. |
+| `retire_date`     | [timestamp][ts] | Optional   | Time that the geography is slated to retire. Once the retire date is passed, new policies can no longer reference it and old policies referencing it should be updated. Retired geographies should continue to be returned in the geographies list. Must be after `effective_date`. Geographies referencing others with `prev_geographies` immediately replace the previous ones. |
 | `prev_geographies` | UUID[]    | Optional   | Unique IDs of prior geographies replaced by this one                                   |
 
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