Add missing doc file and javadoc format fixes

Author: dimitris-athanasiou

client/rest-high-level/src/main/java/org/elasticsearch/client/MachineLearningClient.java

@@ -468,8 +468,8 @@ public PutDatafeedResponse putDatafeed(PutDatafeedRequest request, RequestOption
      * For additional info
      * see <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-put-datafeed.html">ML PUT datafeed documentation</a>
      *
-     * @param request  The request containing the {@link org.elasticsearch.client.ml.datafeed.DatafeedConfig} settings
-     * @param options  Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
+     * @param request The request containing the {@link org.elasticsearch.client.ml.datafeed.DatafeedConfig} settings
+     * @param options Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
      * @param listener Listener to be notified upon request completion
      */
     public void putDatafeedAsync(PutDatafeedRequest request, RequestOptions options, ActionListener<PutDatafeedResponse> listener) {
@@ -485,11 +485,11 @@ public void putDatafeedAsync(PutDatafeedRequest request, RequestOptions options,
      * Gets one or more Machine Learning datafeed configuration info.
      *
      * <p>
-     *     For additional info
-     *     see <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-get-datafeed.html"></a>
-     * </p>
+     * For additional info
+     * see <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-get-datafeed.html">ML GET datafeed documentation</a>
+     *
      * @param request {@link GetDatafeedRequest} Request containing a list of datafeedId(s) and additional options
-     * @param options  Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
+     * @param options Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
      * @return {@link GetDatafeedResponse} response object containing
      * the {@link org.elasticsearch.client.ml.datafeed.DatafeedConfig} objects and the number of jobs found
      * @throws IOException when there is a serialization issue sending the request or receiving the response
@@ -506,11 +506,11 @@ public GetDatafeedResponse getDatafeed(GetDatafeedRequest request, RequestOption
      * Gets one or more Machine Learning datafeed configuration info, asynchronously.
      *
      * <p>
-     *     For additional info
-     *     see <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-get-datafeed.html"></a>
-     * </p>
+     * For additional info
+     * see <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/ml-get-datafeed.html">ML GET datafeed documentation</a>
+     *
      * @param request {@link GetDatafeedRequest} Request containing a list of datafeedId(s) and additional options
-     * @param options  Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
+     * @param options Additional request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
      * @param listener Listener to be notified with {@link GetDatafeedResponse} upon request completion
      */
     public void getDatafeedAsync(GetDatafeedRequest request, RequestOptions options, ActionListener<GetDatafeedResponse> listener) {

docs/java-rest/high-level/ml/get-datafeed.asciidoc

@@ -0,0 +1,56 @@
+[[java-rest-high-x-pack-ml-get-datafeed]]
+=== Get Datafeed API
+
+The Get Datafeed API provides the ability to get {ml} datafeeds in the cluster.
+It accepts a `GetDatafeedRequest` object and responds
+with a `GetDatafeedResponse` object.
+
+[[java-rest-high-x-pack-ml-get-datafeed-request]]
+==== Get Datafeed Request
+
+A `GetDatafeedRequest` object gets can have any number of `datafeedId` entries.
+However, they all must be non-null. An empty list is the same as requesting for all datafeeds.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MlClientDocumentationIT.java[x-pack-ml-get-datafeed-request]
+--------------------------------------------------
+<1> Constructing a new request referencing existing `datafeedIds`, can contain wildcards
+<2> Whether to ignore if a wildcard expression matches no datafeeds.
+ (This includes `_all` string or when no datafeeds have been specified)
+
+[[java-rest-high-x-pack-ml-get-datafeed-execution]]
+==== Execution
+
+The request can be executed through the `MachineLearningClient` contained
+in the `RestHighLevelClient` object, accessed via the `machineLearningClient()` method.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MlClientDocumentationIT.java[x-pack-ml-get-datafeed-execute]
+--------------------------------------------------
+<1> The count of retrieved datafeeds
+<2> The retrieved datafeeds
+
+[[java-rest-high-x-pack-ml-get-datafeed-execution-async]]
+==== Asynchronous Execution
+
+The request can also be executed asynchronously:
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MlClientDocumentationIT.java[x-pack-ml-get-datafeed-execute-async]
+--------------------------------------------------
+<1> The `GetDatafeedRequest` to execute and the `ActionListener` to use when
+the execution completes
+
+The method does not block and returns immediately. The passed `ActionListener` is used
+to notify the caller of completion. A typical `ActionListener` for `GetDatafeedResponse` may
+look like
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MlClientDocumentationIT.java[x-pack-ml-get-datafeed-listener]
+--------------------------------------------------
+<1> `onResponse` is called back when the action is completed successfully
+<2> `onFailure` is called back when some unexpected error occurs