[DOCS] Add block kit messages in Slack connector

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