config.md: clarify destination for mounts

leave room for bind mounts by specifying how dest is interpretted.

config.md

@@ -38,7 +38,7 @@ Additional file systems can be declared as "mounts", declared by the array eleme
 
 * **type** (string, required) Linux, *filesystemtype* argument supported by the kernel are listed in */proc/filesystems* (e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs", "msdos", "proc", "nfs", "iso9660"). Windows: ntfs
 * **source** (string, required) a device name, but can also be a directory name or a dummy. Windows, the volume name that is the target of the mount point. \\?\Volume\{GUID}\ (on Windows source is called target)
-* **destination** (string, required) where the file system is mounted in the container.
+* **destination** (string, required) where the file system is mounted relative to the container rootfs.
 * **options** (string, optional) in the fstab format [https://wiki.archlinux.org/index.php/Fstab](https://wiki.archlinux.org/index.php/Fstab).
 
 *Example (Linux)*