README and SPEC say what docker stop does to requests in progress when the app exits at once (closes #139)

runit stops smallwebwaf and the app at the same time, so the five seconds
smallwebwaf gives requests in progress help only while the app still
answers them. Both documents now say so, say what the client gets when the
app exits at once on SIGTERM, and say what an app does to let its requests
in progress finish.

Model: opus-5-5

SPEC.md

@@ -1324,9 +1324,14 @@ The two processes:
 - A `smallwebwaf` that stops at start, for example on invalid configuration, is
   started again every second and stops again, logging its message each time,
   until the cause is fixed. The health check fails meanwhile.
-- `docker stop` sends SIGTERM to `runsvinit`, which has runit stop both
-  services, each with SIGTERM, and exits once they have stopped. `smallwebwaf`
-  writes its state files as it stops (see "Persistent state").
+- `docker stop` sends SIGTERM to `runsvinit`, which has runit stop both services
+  at once, each with SIGTERM, and exits once they have stopped. `smallwebwaf`
+  stops taking requests, gives those in progress five seconds to finish, and
+  writes its state files as it stops (see "Persistent state"). The requests in
+  progress can finish only while the app still answers. An app that exits at
+  once on SIGTERM ends them: the client gets `502`, or a response cut short if
+  the app had already begun to answer. An app that should finish its requests in
+  progress keeps answering them for up to five seconds after SIGTERM.
 - The container's root filesystem stays writable: runit writes each service's
   status into its directory under `/etc/service`.