manifest: Explicitly base manifest.layers on an empty directory

And point out that layer authors interested in the ownership and other
attributes of the unpacked root directory should explicitly set an
entry for it.  I expect folks will mostly use ./, but have made that
an example in case they want to use other spellings (which unpackers
should respect).

I've used "match"” instead of "be" to allow folks to apply via a union
filesystem or whatever. As long as the finished product is right,
compliance testers and users should be satisfied.

Signed-off-by: W. Trevor King <wking@tremily.us>

manifest.md

@@ -140,9 +140,8 @@ Unlike the [Manifest List](#manifest-list), which contains information about a s
 - **`layers`** *array*
 
     Each item in the array MUST be a [descriptor](descriptor.md).
-    The array MUST have the base image at index 0.
-    Subsequent layers MUST then follow in the order in which they are to be layered on top of each other.
-    The algorithm to create the final unpacked filesystem layout MUST be to first unpack the layer at index 0, then index 1, and so on.
+    The root filesystem MUST match an empty directory onto which the layers have been [applied](layer.md#applying) in the listed order.
+    If none of the layers define an entry for root directory (e.g. `./`), then the [ownership, mode, and other attributes](layer.md#file-attributes) of the final root directory are unspecified.
 
 - **`annotations`** *string-string map*