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

Original created by @intrigeri on 7280 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information