config: Require hook support on Linux and Solaris

Clarifying the language from 28e8f68 (Hooks to linux,solaris and doc
clarification, 2017-05-24, #855), which did not say which platforms
support hooks and which don't.  Without something like this commit,
there's no clear way for config authors to know if their runtime will
support hooks or not, and there was previous agreement that that sort
of ambiguity was not helpful [1].  This also gives normative Markdown
grounding for the Go platform tag added in 28e8f68.

[1]: #472 (comment)

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config.md

@@ -362,9 +362,9 @@ Runtime implementations MAY support any valid values for platform-specific field
 }
 ```
 
-## <a name="configHooks" />Hooks
+## <a name="configHooks" />Linux and Solaris Hooks
 
-Hooks allow for the configuration of custom actions related to the [lifecycle](runtime.md#lifecycle) of the container if supported by the platform.
+For Linux- and Solaris-based systems, the configuration structure supports `hooks` for configuring custom actions related to the [lifecycle](runtime.md#lifecycle) of the container.
 
 * **`hooks`** (object, OPTIONAL) MAY contain any of the following properties:
     * **`prestart`** (array of objects, OPTIONAL) is an array of [pre-start hooks](#prestart).