Add experimental and warnings to vector functions (elastic#42205)

Author: mayya-sharipova

docs/reference/query-dsl/script-score-query.asciidoc

@@ -78,10 +78,18 @@ to be the most efficient by using the internal mechanisms.
 
 [[vector-functions]]
 ===== Functions for vector fields
+
+experimental[]
+
 These functions are used for
 for <<dense-vector,`dense_vector`>>  and
 <<sparse-vector,`sparse_vector`>> fields.
 
+NOTE: During vector functions' calculation, all matched documents are
+linearly scanned. Thus, expect the query time grow linearly 
+with the number of matched documents. For this reason, we recommend
+to limit the number of matched documents with a `query` parameter.
+
 For dense_vector fields, `cosineSimilarity` calculates the measure of
 cosine similarity between a given query vector and document vectors.