[DOCS] Adds descriptions for put and start data frame transforms (#44724)

Author: lcawl

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

@@ -31,6 +31,15 @@ information, see {stack-ov}/security-privileges.html[Security privileges] and
 [[put-data-frame-transform-desc]]
 ==== {api-description-title}
 
+This API defines a {dataframe-transform}, which copies data from source indices,
+transforms it, and persists it into an entity-centric destination index. The
+entities are defined by the set of `group_by` fields in the `pivot` object. You
+can also think of the destination index as a two-dimensional tabular data
+structure (known as a {dataframe}). The ID for each document in the
+{dataframe} is generated from a hash of the entity, so there is a unique row
+per entity. For more information, see
+{stack-ov}/ml-dataframes.html[{dataframe-transforms-cap}].
+
 IMPORTANT:  You must use {kib} or this API to create a {dataframe-transform}.
             Do not put a {dataframe-transform} directly into any
             `.data-frame-internal*` indices using the Elasticsearch index API.

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

@@ -27,6 +27,20 @@ have `view_index_metadata` privileges on the source index for the
 {stack-ov}/security-privileges.html[Security privileges] and
 {stack-ov}/built-in-roles.html[Built-in roles].
 
+[[start-data-frame-transform-desc]]
+==== {api-description-title}
+
+When you start a {dataframe-transform}, it creates the destination index if it
+does not already exist. The `number_of_shards` is set to `1` and the
+`auto_expand_replicas` is set to `0-1`.
+
+The transform deduces the mapping definitions from the source indices. For
+scripted fields, it uses <<dynamic-mapping,dynamic mappings>>. If a field in the
+destination index is created by `scripted_metric` or `bucket_script`
+aggregations, the transform uses dynamic mappings unless a template exists or
+the destination index already exists. Mapping definitions in the destination
+index take precedence over dynamic mappings and templates.
+
 [[start-data-frame-transform-path-parms]]
 ==== {api-path-parms-title}