Clarify contradictory advice on the use of `@PropertySource`

[wilkinsona]

The docs currently say:

A nice way to augment and modify this ordering is to add @PropertySource annotations to your application sources. Classes passed to the SpringApplication static convenience methods and those added using setSources() are inspected to see if they have @PropertySources. If they do, those properties are added to the Environment early enough to be used in all phases of the ApplicationContext lifecycle.

And:

While using @PropertySource on your @SpringBootApplication may seem to be a convenient and easy way to load a custom resource in the Environment, we do not recommend it, because Spring Boot prepares the Environment before the ApplicationContext is refreshed. Any key defined with @PropertySource is loaded too late to have any effect on auto-configuration.

The former is recommending what the latter says is not recommended.

[wilkinsona]

We want to give this one a bit more thought. @PropertySource should work for auto-configuration, but won't work (we don't think) for logging or things like spring.main.* properties.