[doc] describe host-port in .env

Added a short summary in .env.sample,
 copied and shortened from the README.

Reasoning being, operator looking at the config
 might not have the current README open,
 and this setting isn't intuitive from the name.

.env.sample

@@ -25,6 +25,9 @@ COMPOSE_FILE="compose.yml"
 SECRET_SUPERUSER_PASSWORD_VERSION=v1
 
 ## Host-port Mode
+# If enabled, the app container will bind directly to the host's port 64738,
+#  bypassing traefik's entrypoint.
+# This is not recommended for multinode setups.
 #COMPOSE_FILE="$COMPOSE_FILE:compose.host-ports.yml"
 
 #################