config: Do not allow runtimes to ignore properties defined by the spec

Otherwise a runtime could silently ignore a property it didn't want to
implement, which would be confusing for runtime callers [1].  This
closes a potential loophole in the restriction from 766abd6
(runtime.md: Require 'create' to fail if config.json asks for the
impossible, 2016-09-08, opencontainers#559).

[1]: opencontainers#472 (comment)

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

config.md

@@ -415,6 +415,7 @@ Values MAY be an empty string.
 ## Extensibility
 Implementations that are reading/processing this configuration file MUST NOT generate an error if they encounter an unknown property.
 Instead they MUST ignore unknown properties.
+Properties defined for the [target platform](#platform) by the [declared version](#specification-version) of this specification MUST NOT be ignored.
 
 ## Configuration Schema Example