Say what a Valkey recreate actually costs, in the handler comment

The comment claimed every attached app "sees a cold cache". That is not what
happens. RDB persistence is on by default, the recreate is a graceful stop, and
valkey-server writes a final dump.rdb on SIGTERM that the new container loads
back off the bind mount -- so keys survive, which is the part the readme already
gets right and this comment did not.

What does not survive is availability for the seconds the container is down:
Nextcloud keeps its PHP sessions and memcache.locking there, and Yamtrack uses it
as its Celery broker, so those seconds are seconds in which logins and queued
tasks are unreachable. Only a hard kill or a power loss costs changes since the
last save point. That is the warning a future reader needs, and it is what the
nextcloud playbook's own handler comment now points at.

valkey/roles/valkey/handlers/main.yaml

@@ -30,6 +30,12 @@
 # A new drop-in changes the generated unit, so Valkey has to be recreated to join
 # the network — Quadlet has no live equivalent of `podman network connect`.
 #
-# The cost is different from Postgres's: Valkey is a cache, so every attached app
+# The cost is lighter than Postgres's in one way and heavier in another. Lighter:
-# sees a cold cache rather than a period without its database. Correctness is
+# no app is left without its data. RDB persistence is on by default, the recreate
-# unaffected; the first requests after the restart are simply slower.
+# is a graceful stop, and valkey-server writes a final `dump.rdb` on the way down
+# that the new container loads back from the bind mount, so keys survive. Heavier
+# to reason about: two clients do not treat this as a pure cache — Nextcloud keeps
+# its PHP sessions and `memcache.locking` here, and Yamtrack uses it as its Celery
+# broker — so the seconds the container is down are seconds in which logins and
+# queued tasks are unavailable. Only a hard kill or a power loss costs changes
+# since the last save point. See readme.md, "The trade-off".