Document that painless doesn't support the "advanced text scoring" (#24271)

I just spent ages debugging a script I wrote after following the documentation. It was not clear to me that _index is not defined when using painless; if it was mentioned on this page I would have saved myself a lot of time.

docs/reference/modules/scripting/advanced-scripting.asciidoc

@@ -10,6 +10,8 @@ script inside a <<query-dsl-function-score-query,function score query>>.
 Statistics over the document collection are computed *per shard*, not per
 index.
 
+It should be noted that the `_index` variable is not supported in the painless language, but `_index` is defined when using the groovy language.
+
 [float]
 === Nomenclature: