[DOCS] Adds docs about custom URLs in DFA jobs (#2552)

docs/en/stack/ml/df-analytics/index.asciidoc

@@ -8,6 +8,7 @@ include::ml-dfa-classification.asciidoc[leveloffset=+1]
 include::ml-dfa-concepts.asciidoc[leveloffset=+1]
 include::ml-how-dfa-works.asciidoc[leveloffset=+2]
 include::ml-dfa-scale.asciidoc[leveloffset=+2]
+include::ml-dfa-custom-urls.asciidoc[leveloffset=+2]
 include::ml-feature-encoding.asciidoc[leveloffset=+2]
 include::ml-dfa-feature-processors.asciidoc[leveloffset=+2]
 include::ml-feature-importance.asciidoc[leveloffset=+2]

docs/en/stack/ml/df-analytics/ml-dfa-concepts.asciidoc

@@ -7,6 +7,7 @@ This section explains the more complex concepts of the Elastic {ml}
 
 * <<ml-dfa-phases>>
 * <<ml-dfa-scale>>
+* <<ml-dfa-custom-urls>>
 * <<ml-feature-encoding>>
 * <<ml-feature-processors>>
 * <<ml-feature-importance>>

docs/en/stack/ml/df-analytics/ml-dfa-custom-urls.asciidoc

@@ -0,0 +1,84 @@
+[role="xpack"]
+[[ml-dfa-custom-urls]]
+= Adding custom URLs to {dfanalytics-jobs}
+
+You can optionally attach one or more custom URLs to your {dfanalytics-jobs}. 
+These links can direct you to dashboards, the *Discover* app, or 
+external websites. For example, you can define a custom URL that provides a way
+for users to drill down to the source data from a {regression} job. You can 
+create a custom URL during job creation under **Additional settings** in the 
+**Job details** step. Alternatively, you can edit or add new custom URLs in the 
+job list by clicking **Edit** in the **Actions** menu.
+
+[role="screenshot"]
+image::images/ml-dfa-custom-url.png["Creating a custom URL during job creation"width=50%]
+
+When you create or edit an {dfanalytics-job} in {kib}, it simplifies the 
+creation of the custom URLs for {kib} dashboards and the *Discover* app and it 
+enables you to test your URLs. For example:
+
+[role="screenshot"]
+image::images/ml-dfa-custom-url-edit.png["Add a custom URL in {kib}",width=50%]
+
+For each custom URL, you must supply a label. You can also optionally supply a 
+time range. When you link to *Discover* or a {kib} dashboard, you'll have 
+additional options for specifying the pertinent {data-source} or dashboard name 
+and query entities.
+
+
+[discrete]
+[[ml-dfa-url-strings]]
+== String substitution in custom URLs
+
+You can use dollar sign ($) delimited tokens in a custom URL. These tokens are
+substituted for the values of the corresponding fields in the result index. For 
+example, a custom URL might resolve to 
+`discover#/?_g=(time:(from:'$earliest$',mode:absolute,to:'$latest$'))&_a=(filters:!(),index:'4b899bcb-fb10-4094-ae70-207d43183ffc',query:(language:kuery,query:'Carrier:"$Carrier$"'))`. 
+In this case, the pertinent value of the `Carrier` field is passed to the target 
+page when you click the link.
+
+TIP: When you create your custom URL in {kib}, the *Query entities* option is 
+shown only when there are appropriate fields in the index.
+
+The `$earliest$` and `$latest$` tokens pass the beginning and end of the time
+span of the data to the target page. The tokens are substituted with date-time 
+strings in ISO-8601 format. For example, the following API updates a job to add 
+a custom URL that uses `$earliest$` and `$latest$` tokens:
+
+[source,console]
+----------------------------------
+POST _ml/data_frame/analytics/flight-delay-regression/_update
+{
+  "_meta": {
+    "custom_urls": [
+      {
+        "url_name": "flight-delay-regression-results",
+        "url_value": "dashboards#/view/7adfa750-4c81-11e8-b3d7-01146121b73d?_g=(filters:!(),time:('$earliest$',mode:absolute,to:'$latest$'))&_a=(filters:!(),query:(language:kuery,query:''))",
+        "time_range": "1h",
+      }
+    ]
+  }
+}
+----------------------------------
+//TEST[skip:setup:sample_job]
+
+When you click this custom URL, it opens up the *Discover* page and displays 
+source data for the period one hour before and after the date of the default 
+global settings.
+
+[TIP]
+===============================
+* The custom URL links use pop-ups. You must configure your web browser so that 
+it does not block pop-up windows or create an exception for your {kib} URL.
+* When creating a link to a {kib} dashboard, the URLs for dashboards can be very
+long. Be careful of typos, end of line characters, and URL encoding. Also ensure
+you use the appropriate index ID for the target {kib} {data-source}.
+* The dates substituted for `$earliest$` and `$latest$` tokens are in
+ISO-8601 format and the target system must understand this format.
+* If the job performs an analysis against nested JSON fields, the tokens for
+string substitution can refer to these fields using dot notation. For example,
+`$cpu.total$`.
+* {es} source data mappings might make it difficult for the query string to 
+work. Test the custom URL before saving the job configuration to check that it 
+works as expected, particularly when using string substitution.
+===============================