[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}].
+
 When the {dataframe-transform} is created, a series of validations occur to
 ensure its success. For example, there is a check for the existence of the
 source indices and a check that the destination index is not part of the source

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

@@ -30,12 +30,23 @@ have `view_index_metadata` privileges on the source index for the
 [[start-data-frame-transform-desc]]
 ==== {api-description-title}
 
-When a {dataframe-transform} starts, a series of validations occur to ensure its
-success. If you deferred validation when you created the {dataframe-transform},
-they occur when you start the transform--with the exception of privilege checks.
-If the user who created the transform does not have the required privileges on
-the source and destination indices, the transform starts but then fails when
-it attempts the unauthorized operation.
+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.
+
+When the {dataframe-transform} starts, a series of validations occur to ensure
+its success. If you deferred validation when you created the
+{dataframe-transform}, they occur when you start the transform--with the
+exception of privilege checks. If the user who created the transform does not
+have the required privileges on the source and destination indices, the
+transform starts but then fails when it attempts the unauthorized operation.
 
 [[start-data-frame-transform-path-parms]]
 ==== {api-path-parms-title}