Add documentation to @JiltGenerated

src/main/java/org/jilt/JiltGenerated.java

@@ -5,6 +5,25 @@
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
+/**
+ * A marker annotation with retention set to {@link RetentionPolicy#CLASS}
+ * that is added to all Builder classes generated by Jilt.
+ * The only purpose of this annotation is to mark classes it is found on as generated
+ * (meaning, not handwritten) for tools like
+ * <a href="https://www.jacoco.org/jacoco/trunk/doc">JaCoCo</a>.
+ * While Jilt also adds the appropriate {@code @Generated} annotation
+ * ({@code javax.annotation.Generated} when building on Java 8,
+ * or {@code javax.annotation.processing.Generated} when using Java 9 or later)
+ * to all code it generates,
+ * that annotation has only {@link RetentionPolicy#SOURCE source code-level retention},
+ * which means any tool that operates on the compiled class files won't have access to it.
+ * Since {@link JiltGenerated} is retained in the class files,
+ * it allows these tools to identify which classes were generated,
+ * and automatically exclude them from their analysis.
+ * <p>
+ * Other than being a marker, this annotation has no effect,
+ * and can be safely ignored.
+ */
 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.CLASS)
 public @interface JiltGenerated {