config: Explicily make consoleSize unspecified if terminal is false o…

…r unset

The old language is from a502caf (config: Add consoleSize to process,
2016-09-14, opencontainers#563), where nobody commented on the "if attached" wording
[1].  But reading the old line now, it's not clear to me what
consoleSize means when terminal is not true.

This commit explicitly declares consoleSize unspecified in that
condition, so runtimes are free to do what they want short of erroring
out.  I considered making the property undefined or requiring it to be
unset, but those seemed too strict given our permissive "MUST ignore
unknown properties" extensibility requirement.

[1]: opencontainers#563

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

config.md

@@ -137,7 +137,8 @@ For all platform-specific configuration values, the scope defined below in the [
 
 * **`terminal`** (bool, OPTIONAL) specifies whether a terminal is attached to that process, defaults to false.
     As an example, if set to true on Linux a pseudoterminal pair is allocated for the container process and the pseudoterminal slave is duplicated on the container process's [standard streams][stdin.3].
-* **`consoleSize`** (object, OPTIONAL) specifies the console size in characters of the terminal if attached, containing the following properties:
+* **`consoleSize`** (object, OPTIONAL) specifies the console size in characters of the terminal.
+    `consoleSize` is unspecified if `terminal` is `false` or unset.
     * **`height`** (uint, REQUIRED)
     * **`width`** (uint, REQUIRED)
 * **`cwd`** (string, REQUIRED) is the working directory that will be set for the executable.