Update fields description (#28483)

Author: pamelafox

specification/search/data-plane/Azure.Search/preview/2024-03-01-Preview/searchindex.json

@@ -1863,7 +1863,7 @@
         },
         "fields": {
           "type": "string",
-          "description": "Vector Fields of type Collection(Edm.Single) to be included in the vector searched."
+          "description": "The comma-separated list of field names to be included in the vector search. Each of the fields must be a valid vector type, such as Collection(Edm.Single). If multiple fields are given, the vector will be used to find the closest matches across all fields."
         },
         "exhaustive": {
           "type": "boolean",