docs: graceful shutdown docs

Considered the explanations and use cases on:

- https://github.com/yesodweb/wai/issues/853
- https://github.com/PostgREST/postgrest/pull/4580

docs/postgrest.dict

@@ -1,5 +1,6 @@
 personal_ws-1.1 en 0 utf-8
 api
+autoscaling
 API's
 APIs
 APISIX
@@ -34,6 +35,7 @@ DevOps
 Dramatiq
 dockerize
 enum
+ECS
 Enums
 Entra
 eq

docs/references/http_server.rst

@@ -4,3 +4,15 @@ HTTP Server
 ###########
 
 The HTTP server is provided by `Warp <https://aosabook.org/en/posa/warp.html>`_.
+
+Graceful shutdown
+-----------------
+
+PostgREST uses Warp's graceful shutdown, when a ``SIGTERM`` is received:
+
+- It stops accepting new requests.
+- Allows requests that are already in progress to finish.
+- Closes idle ``Keep-Alive`` connections instead of waiting for them to expire.
+- Responses sent during shutdown indicate that the connection should not be reused (e.g. for HTTP/1.x, it sends ``Connection: close``).
+
+This allows PostgREST to shut down promptly without interrupting in-flight requests. Useful for zero-downtime upgrades and autoscaling/load-balancing under cloud environments (AWS ECS, Kubernetes).