Add notes about issue reported where unset() breaks when removing attachments

src/compose.php

@@ -659,6 +659,8 @@ elseif (isset($sigappend)) {
         foreach($delete as $index) {
             if (!empty($composeMessage->entities) && isset($composeMessage->entities[$index])) {
                 $composeMessage->entities[$index]->purgeAttachments();
+                // FIXME: one person reported that unset() didn't do anything at all here, so this is a work-around... but it triggers PHP notices if the unset() doesn't work, which should be fixed... but bigger question is if unset() doesn't work here, what about everywhere else? Anyway, uncomment this if you think you need it
+                //$composeMessage->entities[$index] = NULL;
                 unset ($composeMessage->entities[$index]);
             }
         }