Merge pull request opencontainers#681 from wking/valid-values

config: Move valid-value rules to their own section

config.md

@@ -324,7 +324,6 @@ For Windows based systems the user structure has the following fields:
 
 [**`platform.os`**](#platform) is used to specify platform-specific configuration.
 Runtime implementations MAY support any valid values for platform-specific fields as part of this configuration.
-Implementations MUST error out when invalid values are encountered and MUST generate an error message and error out when encountering valid values it chooses to not support.
 
 * **`linux`** (object, OPTIONAL) [Linux-specific configuration](config-linux.md).
   This MAY be set if **`platform.os`** is `linux` and MUST NOT be set otherwise.
@@ -444,6 +443,11 @@ Values MAY be an empty string.
 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.
 
+## Valid values
+
+Implementations that are reading/processing this configuration file MUST generate an error when invalid or unsupported values are encountered.
+Unless support for a valid value is explicitly required, runtimes MAY choose which subset of the valid values it will support.
+
 ## Configuration Schema Example
 
 Here is a full example `config.json` for reference.