[DOCS] Clarifies data transform source definition (#44504)

docs/reference/data-frames/apis/preview-transform.asciidoc

@@ -40,7 +40,18 @@ on all the current data in the source index.
 ==== {api-request-body-title}
 
 `source`::
-  (Required, object) The source index or index pattern.
+  (Required, object) The source configuration, which has the following
+  properties:
+  
+  `index`:::
+    (Required, string or array) The _source indices_ for the
+    {dataframe-transform}. It can be a single index, an index pattern (for
+    example, `"myindex*"`), or an array of indices (for example,
+    `["index1", "index2"]`).
+
+  `query`:::
+    (Optional, object) A query clause that retrieves a subset of data from the
+    source index. See <<query-dsl>>.
 
 `pivot`::
   (Required, object) Defines the pivot function `group by` fields and the

docs/reference/data-frames/apis/put-transform.asciidoc

@@ -71,8 +71,18 @@ IMPORTANT:  You must use {kib} or this API to create a {dataframe-transform}.
   reduce the data. See <<data-frame-transform-pivot>>.
 
 `source`::
-  (Required, object) The source configuration, which consists of `index` and 
-  optionally a `query`. See <<data-frame-transform-source>>.
+  (Required, object) The source configuration, which has the following
+  properties:
+  
+  `index`:::
+    (Required, string or array) The _source indices_ for the
+    {dataframe-transform}. It can be a single index, an index pattern (for
+    example, `"myindex*"`), or an array of indices (for example,
+    `["index1", "index2"]`).
+    
+    `query`:::
+      (Optional, object) A query clause that retrieves a subset of data from the
+      source index. See <<query-dsl>>.
   
 `sync`::
   (Optional, object) Defines the properties required to run continuously.

docs/reference/data-frames/apis/transformresource.asciidoc

@@ -61,8 +61,10 @@ you create a {dataframe-transform}, you must define its source.
 [[data-frame-transform-source-properties]]
 ===== {api-definitions-title}
 
-`index`:: 
-  (array) The _source index_ for the {dataframe-transform}.
+`index`::
+  (string or array) The _source indices_ for the {dataframe-transform}. It can
+  be a single index, an index pattern (for example, `"myindex*"`), or an array
+  of indices (for example, `["index1", "index2"]`).
 
 `query`::
   (object) A query clause that retrieves a subset of data from the source index.