sklad: use textarea placeholder also for editing

cmd/sklad/container.tmpl

@@ -36,9 +36,9 @@
 </header>
 
 <form method=post action="container?id={{ .Container.Id }}">
-<textarea name=description rows="{{ max 5 (lines .Container.Description) }}">
-{{ .Container.Description }}
-</textarea>
+<textarea name=description rows="{{ max 5 (lines .Container.Description) }}"
+	placeholder="Popis obalu nebo jeho obsahu">
+{{ .Container.Description }}</textarea>
 <footer>
 	<div>
 		<label for=series>Řada:</label>