config: Consistent Markdown/Go/JSON-Schema wording for 'root'

I've also added our usual:

  (<type>, <required|optional>)

to the Markdown so folks can see that this is a required object.

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

config.md

@@ -22,7 +22,7 @@ For example, if an implementation is compliant with version 1.0.1 of the spec, i
 
 ## Root Configuration
 
-Each container has exactly one *root filesystem*, specified in the *root* object:
+**`root`** (object, required) configures the container's root filesystem.
 
 * **`path`** (string, required) Specifies the path to the root filesystem for the container.
   A directory MUST exist at the path declared by the field.

schema/config-schema.json

@@ -56,7 +56,7 @@
             }
         },
         "root": {
-            "description": "The path to the root filesystem for the container.",
+            "description": "Configures the container's root filesystem.",
             "id": "https://opencontainers.org/schema/bundle/root",
             "type": "object",
             "properties": {

specs-go/config.go

@@ -10,7 +10,7 @@ type Spec struct {
 	Platform Platform `json:"platform"`
 	// Process configures the container process.
 	Process Process `json:"process"`
-	// Root is the root information for the container's filesystem.
+	// Root configures the container's root filesystem.
 	Root Root `json:"root"`
 	// Hostname is the container's host name.
 	Hostname string `json:"hostname,omitempty"`