config: Replace "SHOULD consider" for 'rootfs' path value

SHOULD means that [1]:

  ... there may exist valid reasons in particular circumstances to
  ignore a particular item, but the full implications must be
  understood and carefully weighed before choosing a different course.

So consideration is already baked into the definition, and we can just
say "SHOULD be" and warn folks whenever they use a value other than
'rootfs'.

Also break this into its own paragraph, because the value
recommendation is orthogonal to and less important than the value
semantics covered in the preceding sentences.

[1]: https://tools.ietf.org/html/rfc2119#section-3

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

Author: wking

config.md

@@ -29,7 +29,8 @@ 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`.
+
+    The value SHOULD be the conventional `rootfs`.
 
     On Windows, for Windows Server Containers, this field is REQUIRED. For Hyper-V Containers, this field MUST be omitted.