Expand @UnstableApi javadoc with how to request stable API additions

Issue: androidx#503

#minor-release

PiperOrigin-RevId: 547143885

libraries/common/src/main/java/androidx/media3/common/util/UnstableApi.java

@@ -41,6 +41,15 @@
  * work during upgrades. However it is generally inadvisable for libraries (which get included on
  * users' CLASSPATHs, outside the library developers' control) to do so.
  *
+ * <h2>Requesting additions to the stable API</h2>
+ *
+ * The Media3 stable API (i.e. those public API symbols that are not annotated with this annotation)
+ * is designed to allow developers to achieve common media-related tasks. If you have a use-case
+ * that you are unable to achieve using the stable API, and think you should be able to, please file
+ * an issue on our <a href="https://github.com/androidx/media/issues">GitHub issue tracker</a> with
+ * the full context of what you're doing, and what symbols you would need to be part of the stable
+ * API. We will consider each request on a case-by-case basis.
+ *
  * <h2>Opting in to use unstable APIs</h2>
  *
  * <p>By default usages of APIs annotated with this annotation generate lint errors in Gradle and