Update the AutoBuilder documentation about stability

RELNOTES=n/a
PiperOrigin-RevId: 368677272
diff --git a/value/src/main/java/com/google/auto/value/AutoBuilder.java b/value/src/main/java/com/google/auto/value/AutoBuilder.java
index 481f6dc..cfaa7ce 100644
--- a/value/src/main/java/com/google/auto/value/AutoBuilder.java
+++ b/value/src/main/java/com/google/auto/value/AutoBuilder.java
@@ -21,8 +21,8 @@
 import java.lang.annotation.Target;
 
 /**
- * Specifies that the annotated interface or abstract class should be implemented as a builder;
- * THIS IS NOT YET READY FOR USE.
+ * Specifies that the annotated interface or abstract class should be implemented as a builder.
+ * This is still unstable; uses outside Google may break.
  *
  * <p>A simple example:
  *