config: Explicily make consoleSize unspecified if terminal is false or unset

The old language is from a502caf6 (config: Add consoleSize to process,
2016-09-14, #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 ignored in that condition.
I'd rather have made it unspecified, so runtimes are free to do what
they want short of erroring out, but Michael wanted the more specific
"ignored" [2].  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]: https://github.com/opencontainers/runtime-spec/pull/563
[2]: https://github.com/opencontainers/runtime-spec/pull/863#discussion_r119682760

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.
+    Runtimes MUST ignore `consoleSize` 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.