config: Add platform-specific entry for 'solaris'

Fixup for 7c9daeb (Introducing Solaris in OCI, 2016-04-25, opencontainers#411) along
the lines of b373a15 (config: Split platform-specific configuration
into its own section, 2016-05-02, opencontainers#414).

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

config.md

@@ -207,6 +207,8 @@ _Note: For Solaris, uid and gid specify the uid and gid of the process inside th
 
 * **`linux`** (object, optional) [Linux-specific configuration](config-linux.md).
   This should only be set if **`platform.os`** is `linux`.
+* **`solaris`** (object, optional) [Solaris-specific configuration](config-solaris.md).
+  This should only be set if **`platform.os`** is `solaris`.
 
 ### Example (Linux)