docs: Update the Javadoc of ObsoleteApi.java (#3088)

Update the Javadoc of ObsoleteApi.java. It's preferred to provide
reasons and alternatives directly in the description, rather than links,
as links are not stable.

api-common-java/src/main/java/com/google/api/core/ObsoleteApi.java

@@ -60,6 +60,6 @@
 })
 @Documented
 public @interface ObsoleteApi {
-  /** Context information such as links to a discussion thread, tracking issue, etc. */
+  /** Context information such as reasons and alternatives. */
   String value();
 }