Document modern alternatives to AutoValue.

If you are using Kotlin or Java ≥16 then AutoValue is no longer usually what you want.

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PiperOrigin-RevId: 430755878

value/README.md

@@ -21,5 +21,12 @@ less code and less room for error, while **not restricting your freedom** to
 code almost any aspect of your class exactly the way you want it.
 
 For more information, consult the
-[detailed
-documentation](userguide/index.md)
+[detailed documentation](userguide/index.md).
+
+**Note:** If you are using Kotlin then its
+[data classes](https://kotlinlang.org/docs/data-classes.html) are usually more
+appropriate than AutoValue. Likewise, if you are using a version of Java that
+has [records](https://docs.oracle.com/en/java/javase/16/language/records.html),
+then those are usually more appropriate. You can still use
+[AutoBuilder](userguide/autobuilder.md) to make builders for data classes or
+records.

value/userguide/index.md

@@ -31,6 +31,14 @@ AutoValue provides an easier way to create immutable value classes, with a lot
 less code and less room for error, while **not restricting your freedom** to
 code almost any aspect of your class exactly the way you want it.
 
+**Note**: If you are using Kotlin then its
+[data classes](https://kotlinlang.org/docs/data-classes.html) are usually more
+appropriate than AutoValue. Likewise, if you are using a version of Java that
+has [records](https://docs.oracle.com/en/java/javase/16/language/records.html),
+then those are usually more appropriate. You can still use
+[AutoBuilder](userguide/autobuilder.md)--> to make builders for data classes or
+records.
+
 This page will walk you through how to use AutoValue. Looking for a little more
 persuasion? Please see [Why AutoValue?](why.md).
 
@@ -279,4 +287,4 @@ How do I...
 *   ... [create a **pretty string** representation?](howto.md#toprettystring)
 
 
-<!-- TODO(kevinb): should the above be only a selected subset? -->
+<!-- TODO(kevinb): should the above be only a selected subset?

value/userguide/why.md

@@ -1,8 +1,10 @@
 # Why use AutoValue?
 
 
-AutoValue is the only solution to the value class problem in Java having all of
-the following characteristics:
+In versions of Java preceding
+[records](https://docs.oracle.com/en/java/javase/16/language/records.html),
+AutoValue is the only solution to the value class problem having all of the
+following characteristics:
 
 *   **API-invisible** (callers cannot become dependent on your choice to use it)
 *   No runtime dependencies