[Discover][docs] Add searchFieldsFromSource description (#90980)

Co-authored-by: James Rodewig <40268737+jrodewig@users.noreply.github.com>
Co-authored-by: gchaps <33642766+gchaps@users.noreply.github.com>

docs/management/advanced-options.asciidoc

@@ -258,8 +258,12 @@ Highlights results in *Discover* and saved searches on dashboards. Highlighting
 slows requests when working on big documents.
 
 [[doctable-legacy]]`doc_table:legacy`:: 
-Control the way the Discover's table looks and works. Set this property to `true` to revert to the legacy implementation.
+Controls the way the document table looks and works. Set this property to `true` to revert to the legacy implementation.
 
+[[discover-searchFieldsFromSource]]`discover:searchFieldsFromSource`::
+Load fields from the original JSON {ref}/mapping-source-field.html[`_source`].
+When disabled, *Discover* loads fields using the {es} search API's
+{ref}/search-fields.html#search-fields-param[`fields`] parameter.
 
 [float]
 [[kibana-ml-settings]]