Change help text for StructuredConfig.CONFIG property

Current help text 'serialized to the chosen format' is misleading
as it only converts to JSON. Hooks have the responsibility to
convert to the required format and use it.

Change-Id: Ic9df130193982c3a89330931812512e055a49029
Closes-Bug: #1339464
Merge "Change help text for StructuredConfig.CONFIG property"