[8.13] [DOCS] Add block kit messages in Slack connector (#177677) (#178269)

# Backport

This will backport the following commits from `main` to `8.13`:
- [[DOCS] Add block kit messages in Slack connector
(#177677)](https://github.com/elastic/kibana/pull/177677)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Lisa
Cawley","email":"lcawley@elastic.co"},"sourceCommit":{"committedDate":"2024-03-08T01:16:30Z","message":"[DOCS]
Add block kit messages in Slack connector
(#177677)","sha":"0fcc69f59d066c24a2203a0af3e35694725dee56","branchLabelMapping":{"^v8.14.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:skip","Team:ResponseOps","docs","Feature:Actions/ConnectorTypes","v8.13.0","v8.14.0"],"title":"[DOCS]
Add block kit messages in Slack
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Add block kit messages in Slack connector
(#177677)","sha":"0fcc69f59d066c24a2203a0af3e35694725dee56"}},"sourceBranch":"main","suggestedTargetBranches":["8.13"],"targetPullRequestStates":[{"branch":"8.13","label":"v8.13.0","branchLabelMappingKey":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"},{"branch":"main","label":"v8.14.0","branchLabelMappingKey":"^v8.14.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/177677","number":177677,"mergeCommit":{"message":"[DOCS]
Add block kit messages in Slack connector
(#177677)","sha":"0fcc69f59d066c24a2203a0af3e35694725dee56"}}]}]
BACKPORT-->

Co-authored-by: Lisa Cawley <lcawley@elastic.co>

docs/management/connectors/action-types/slack.asciidoc

@@ -38,14 +38,19 @@ For Slack setup details, go to <<configuring-slack>>.
 === Test connectors
 
 You can test connectors as you're creating or editing the connector in {kib}.
-For example: 
+For the webhook type of connector, its message text cannot contain Markdown, images, or other advanced formatting:
 
 [role="screenshot"]
 image::management/connectors/images/slack-webhook-params.png[Slack webhook connector test]
 // NOTE: This is an autogenerated screenshot. Do not edit it directly.
 
-Slack connector actions have message text, which cannot contain Markdown, images, or other advanced formatting.
-For the web API type of connector, you must also choose one of the channel IDs.
+For the web API type of connector, you must choose one of the channel IDs.
+You can then test either plain text or block kit messages:
+
+[role="screenshot"]
+image::management/connectors/images/slack-api-connector-test.png[Slack web API connector test]
+
+After you add block kit messages you can click a link to preview them in the Slack block kit builder.
 
 [float]
 [[slack-connector-networking-configuration]]

x-pack/plugins/actions/docs/openapi/bundled.json

@@ -6102,7 +6102,7 @@
               },
               "text": {
                 "type": "string",
-                "description": "The Slack message text, which cannot contain Markdown, images, or other advanced formatting.",
+                "description": "The Slack message text. If it is a Slack webhook connector, the text cannot contain Markdown, images, or other advanced formatting. If it is a Slack web API connector, it can contain either plain text or block kit messages.\n",
                 "minLength": 1
               }
             }
@@ -7063,4 +7063,4 @@
       }
     }
   }
-}
+}

x-pack/plugins/actions/docs/openapi/bundled.yaml

@@ -4278,7 +4278,8 @@ components:
                 type: string
             text:
               type: string
-              description: The Slack message text, which cannot contain Markdown, images, or other advanced formatting.
+              description: |
+                The Slack message text. If it is a Slack webhook connector, the text cannot contain Markdown, images, or other advanced formatting. If it is a Slack web API connector, it can contain either plain text or block kit messages.
               minLength: 1
     run_connector_subaction_validchannelid:
       title: The validChannelId subaction

x-pack/plugins/actions/docs/openapi/bundled_serverless.json

@@ -4418,4 +4418,4 @@
       }
     }
   }
-}
+}

x-pack/plugins/actions/docs/openapi/components/schemas/run_connector_subaction_postmessage.yaml

@@ -33,6 +33,9 @@ properties:
           type: string
       text:
         type: string
-        description: The Slack message text, which cannot contain Markdown, images, or other advanced formatting.
+        description: >
+          The Slack message text.
+          If it is a Slack webhook connector, the text cannot contain Markdown, images, or other advanced formatting.
+          If it is a Slack web API connector, it can contain either plain text or block kit messages.
         minLength: 1