Document that boot in older VirtualBox is broken in Tails 1.1
This bug is described in #7232 (closed). The proper fix will be #7173 (closed). In the meantime, we must document the limitation in the known issues.
Feature Branch: doc/7280-VirtualBox-vs-syslinux-6
Parent Task: #5739 (closed)
Original created by @intrigeri on 7280 (Redmine)