Similarity is applicable for text & keyword fields

Note that similarity is only applicable for text and keyword field types.

docs/reference/index-modules/similarity.asciidoc

@@ -5,6 +5,8 @@ A similarity (scoring / ranking model) defines how matching documents
 are scored. Similarity is per field, meaning that via the mapping one
 can define a different similarity per field.
 
+Similarity is only applicable for text type and keyword type fields.
+
 Configuring a custom similarity is considered an expert feature and the
 builtin similarities are most likely sufficient as is described in
 <<similarity>>.