[SPR-8386][SPR-8387] Refined logging regarding detection of default resource locations and default configuration classes.

- Removed generatesDefaults() and supports() from the SmartContextLoader SPI and modified the DelegatingSmartContextLoader algorithm accordingly.
- DelegatingSmartContextLoader no longer operates on a list of candidate loaders but rather on explicit instances of GenericXmlContextLoader and AnnotationConfigContextLoader.
- Updated Javadoc regarding DelegatingSmartContextLoader as the new default loader.
- Updated and polished Javadoc regarding changes in 3.1.
- Now enforcing @ContextConfiguration's restriction on declaring locations or classes but not both.

[SPR-8387] Fixed logic errors in DelegatingSmartContextLoader.processContextConfiguration() and ContextConfigurationAttributes.hasResources().

[SPR-8387] Fleshing out the implementation of processContextConfiguration() in DelegatingSmartContextLoader.

[SPR-8387] Introduced supports(MergedContextConfiguration) method in the SmartContextLoader SPI; updated existing loaders accordingly; and fleshed out implementation of and tests for the new DelegatingSmartContextLoader.