[DOCS] Add transform limitation for underscore field names (#86195)

docs/reference/transform/limitations.asciidoc

@@ -22,6 +22,14 @@ categories:
 [[transform-config-limitations]]
 == Configuration limitations
 
+[discrete]
+[[transforms-underscore-limitation]]
+=== Field names prefixed with underscores are omitted from latest {transforms}
+
+If you use the `latest` type of {transform} and the source index has field names
+that start with an underscore (_) character, they are assumed to be internal
+fields. Those fields are omitted from the documents in the destination index.
+
 [discrete]
 [[transforms-ccs-limitation]]
 === {transforms-cap} support {ccs} if the remote cluster is configured properly