Document that auto-configuration classes should be identified using their binary names

[wilkinsona]

When an auto-configuration class is listed in AutoConfiguration.imports or referenced as a String using the name attribute of @AutoConfigureBefore or @AutoConfigureAfter, it should be identified using its binary name. For top-level classes, this makes no difference (for example, com.example.ExampleAutoConfiguration). For nested classes, this requires the use of a $ separator rather than a . beyond the top-level class (for example com.example.Outer$NestedAutoConfiguration).