Add a comment to jdk.internal.PreviewFeature.Feature.TEXT_BLOCKS enum to indicate why it remains in the code.

"This enum constant is not used in the JDK 15 codebase, but exists to support the bootcycle build of JDK 15. The bootcycle build of JDK 15 is performed with JDK 14 and the PreviewFeature type from JDK 15. Since the JDK 14 codebase uses the enum constant, it is necessary for PreviewFeature in JDK 15 to declare the enum constant."