config: Move valid-value rules to their own section

This wording just landed via 718f9f3 (origin/pr/673) minor narrative
cleanup regarding config compatibility, 2017-01-30, opencontainers#673), but the
rule is generic and not unique to platform-specific properties.

Also adjust the wording somewhat to match the more established wording
from the "Extensibility" section.

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

Author: wking

config.md

@@ -292,7 +292,6 @@ 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.
@@ -416,6 +415,11 @@ 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.