bundle: Move 'rootfs' recommendation to config.md

Anyone setting 'rootfs.path' should be aware of this advice,
regardless of whether they're the ones composing the bundle or not.

Use the RFC 2119 "SHOULD" (vs. the old lowercase "should") for this
recommendation.  The SHOULD semantics make sense and using SHOULD
avoids confusing readers ("did they mean to SHOULD this?").

Also drop the "While the name of this directory may be arbitrary"
caveat, because SHOULD already implies "but you can pick another
directory name if you want".

Note that the "MUST be present in a single directory" line which is
still in bundle.md forbids you from picking "foo/bar", "../foo", or
other paths that do not point at a direct child of the bundle
directory.  I don't like that direct-child restriction [1], but I'm
not touching it in this commit.

[1]: #469

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

Author: wking

bundle.md

@@ -15,8 +15,6 @@ This REQUIRED file MUST reside in the root of the bundle directory and MUST be n
 See [`config.json`](config.md) for more details.
 
 2. <a name="containerFormat02" />A directory representing the root filesystem of the container.
-    While the name of this directory may be arbitrary, users should consider using a conventional name, such as `rootfs`.
-
     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.

config.md

@@ -29,6 +29,7 @@ For example, if a configuration is compliant with version 1.1 of this specificat
 **`root`** (object, REQUIRED) specifies the container's root filesystem.
 
 * **`path`** (string, OPTIONAL) Specifies the path to the root filesystem for the container. The path is either an absolute path or a relative path to the bundle.
+    Users SHOULD consider using a conventional name, such as `rootfs`.
 
     On Windows, for Windows Server Containers, this field is REQUIRED. For Hyper-V Containers, this field MUST be omitted.