update NDCollector doc (#12349)

* explain the details for Scala Experimental

Author: yzhliu

scala-package/core/src/main/scala/org/apache/mxnet/NDArrayCollector.scala

@@ -133,6 +133,10 @@ class NDArrayCollector private(private val autoDispose: Boolean = true,
    * If the return type of scope is <em>NDArray</em> or <em>NDArrayFuncReturn</em>,
    * it is smart enough NOT to collect or dispose the returned NDArray. <br />
    * However in other cases, it is users' responsibility NOT to leak allocated NDArrays outside.
+   * <br />
+   * We might switch to try -with-resources statement (by AutoCloseable in Java 1.7+)
+   * and deprecate this method later, thus it is marked as Experimental.
+   *
    * @param codeBlock code block to be executed within the scope.
    * @tparam T return type of the function <em>codeBlock</em>.
    * @return The result of function <em>codeBlock</em>.

scala-package/core/src/main/scala/org/apache/mxnet/annotation/Experimental.scala

@@ -21,7 +21,7 @@ import java.lang.annotation.{ElementType, Retention, Target, _}
 
 /**
   * Experimental: there is a comparably high chance that
-  * the API will undergo some kind of changes
+  * the API will be changed or removed.
   */
 @Retention(RetentionPolicy.RUNTIME)
 @Target(Array(ElementType.TYPE, ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER,