[unpackfs] Expand on the documentation

src/modules/unpackfs/unpackfs.conf

@@ -11,14 +11,14 @@
 ---
 # Each list item is unpacked, in order, to the target system.
 #
-# Each list item has the following attributes:
-#   source: path relative to the live / intstalling system to the image
-#   sourcefs: the type of the source files; valid entries are
-#       - *ext4* (copies the filesystem contents)
-#       - *squashfs* (unsquashes)
-#       - *file* (copies a file or directory)
+# Each list item has the following **mandatory** attributes:
+#   - *source* path relative to the live / intstalling system to the image
+#   - *sourcefs* the type of the source files; valid entries are
+#       - `ext4` (copies the filesystem contents)
+#       - `squashfs` (unsquashes)
+#       - `file` (copies a file or directory)
 #       - (may be others if mount supports it)
-#   destination: path relative to rootMountPoint (so in the target
+#   - *destination* path relative to rootMountPoint (so in the target
 #       system) where this filesystem is unpacked. It may be an
 #       empty string, which effectively is / (the root) of the target
 #       system.
@@ -26,13 +26,18 @@
 # EXAMPLES
 #
 # Usually you list a filesystem image to unpack; you can use
-# squashfs or an ext4 image.
+# squashfs or an ext4 image. An empty destination is equivalent to "/",
+# the root of the target system. The destination directory must exist
+# in the target system.
 #
 #   -   source: "/path/to/filesystem.sqfs"
 #       sourcefs: "squashfs"
 #       destination: ""
 #
-# Multiple entries are unpacked in-order
+# Multiple entries are unpacked in-order; if there is more than one
+# item then only the first must exist beforehand -- it's ok to
+# create directories with one unsquash and then to use those
+# directories as a target from a second unsquash.
 #
 #   -   source: "/path/to/another/filesystem.img"
 #       sourcefs: "ext4"