bundle: DRYer root.path entry

Instead of repeating the config.md conditions, just make the linkage
more clear.  Now the chain is:

1. Bundles REQUIRE a config.json which is a bundle artifact.
2. If that config.json has a root.path entry (as specified in
   config.md), then add the referenced directory to the set of bundle
   artifacts.  The config.md requirements include "If defined, a
   directory MUST exist at the path declared by the field".
3. Apply the "MUST all be present in a single directory" condition to
   all bundle artifacts.  I don't like that direct-child restriction
   [1], but I'm not touching it in this commit.

So these are the same requirements as before this commit, but with
less redundancy and fewer words.

[1]: opencontainers/runtime-spec#469

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

bundle.md

@@ -14,13 +14,7 @@ This includes the following artifacts:
     This REQUIRED file MUST reside in the root of the bundle directory and MUST be named `config.json`.
     See [`config.json`](config.md) for more details.
 
-2. <a name="containerFormat02" />A directory representing the root filesystem of the container.
-    On Windows, for Windows Server containers, this directory is REQUIRED.
-    For Hyper-V containers, it MUST be omitted.
-
-    On all other platforms, this field is REQUIRED.
-
-    If set, this directory MUST be referenced by [`root`](config.md#root) within the `config.json` file.
+2. <a name="containerFormat02" />container's root filesystem: the directory referenced by [`root.path`](config.md#root), if that property is set in `config.json`.
 
 When supplied, while these artifacts MUST all be present in a single directory on the local filesystem, that directory itself is not part of the bundle.
 In other words, a tar archive of a *bundle* will have these artifacts at the root of the archive, not nested within a top-level directory.