Clarify when SpEL expressions in application properties are evaluated

[wilkinsona]

The documentation currently states that @Value will evaluate SpEL expressions and that "while you can write a SpEL expression in @Value, such expressions are not processed from application property files". This second point isn't worded as precisely as it could be. It is trying to say that SpEL expressions are not processed when parsing application properties and populating the environment. However, if a property's value contains a SpEL expression it will be evaluated when consumed via @Value, irrespective of the property's origin.