Add comments to `wait-for-postgres.sh`

Add additional comment to explain code

compose/startup-order.md

@@ -68,14 +68,25 @@ script:
   set -e
   
   host="$1"
+  # Shift arguments with mapping:
+  # - $0 => $0
+  # - $1 => <discarded>
+  # - $2 => $1
+  # - $3 => $2
+  # - ...
+  # This is done for `exec "$@"` below to work correctly
   shift
   
+  # Login for user (`-U`) and once logged in execute quit ( `-c \q` )
+  # If we can not login sleep for 1 sec
   until PGPASSWORD=$POSTGRES_PASSWORD psql -h "$host" -U "postgres" -c '\q'; do
     >&2 echo "Postgres is unavailable - sleeping"
     sleep 1
   done
   
   >&2 echo "Postgres is up - executing command"
+  # Print and execute all other arguments starting with `$1`
+  # So `exec "$1" "$2" "$3" ...`
   exec "$@"
   ```