[DOCS] Update RRF tech preview statement (#97851)

* [DOCS] Update RRF tech preview statement

* Add 'rank' and 'sub_searches'

docs/reference/search/rrf.asciidoc

@@ -1,7 +1,7 @@
 [[rrf]]
 === Reciprocal rank fusion
 
-preview::[]
+preview::["This functionality is in technical preview and may be changed or removed in a future release. The syntax will likely change before GA. Elastic will apply best effort to fix any issues, but features in technical preview are not subject to the support SLA of official GA features."]
 
 https://plg.uwaterloo.ca/~gvcormac/cormacksigir09-rrf.pdf[Reciprocal rank fusion (RRF)]
 is a method for combining multiple result sets with different relevance

docs/reference/search/search.asciidoc

@@ -556,7 +556,8 @@ Period of time used to extend the life of the PIT.
 [[request-body-rank]]
 `rank`::
 preview:[]
-This param is in technical preview and may change in the future.
+This param is in technical preview and may change in the future. The syntax will
+likely change before GA.
 +
 (Optional, object)
 Defines a method for combining and ranking result sets from
@@ -722,7 +723,8 @@ aggregation for its associated searches. You can retrieve these stats using the
 [[request-body-sub-searches]]
 `sub_searches`::
 preview:[]
-This param is in technical preview and may change in the future.
+This param is in technical preview and may change in the future. The syntax will
+likely change before GA.
 +
 (Optional, array of objects)
 An array of `sub_search` objects where each `sub_search` is evaluated