config: Consistent Markdown/Go wording for 'hooks'

The new wording isn't particularly close to either of the old
wordings, but it reads more clearly to me.  I've also added our usual:

  (<type>, <required|optional>)

to the Markdown so folks can see that this is an optional object
(although see [1] for a more complete version).

[1]: opencontainers#427
     Subject: config: Explicitly list 'hooks' as optional

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

config.md

@@ -251,6 +251,7 @@ _Note: For Solaris, uid and gid specify the uid and gid of the process inside th
 
 ## Hooks
 
+**`hooks`** (object, optional) configures callbacks for container lifecycle events.
 Lifecycle hooks allow custom events for different points in a container's runtime.
 Presently there are `Prestart`, `Poststart` and `Poststop`.
 

specs-go/config.go

@@ -16,7 +16,7 @@ type Spec struct {
 	Hostname string `json:"hostname,omitempty"`
 	// Mounts configures additional mounts (on top of Root).
 	Mounts []Mount `json:"mounts,omitempty"`
-	// Hooks are the commands run at various lifecycle events of the container.
+	// Hooks configures callbacks for container lifecycle events.
 	Hooks Hooks `json:"hooks"`
 	// Annotations is an unstructured key value map that may be set by external tools to store and retrieve arbitrary metadata.
 	Annotations map[string]string `json:"annotations,omitempty"`