Add file extensions for vector search for preload (elastic#96955)

In this tuning guide we mentioned preload to warm up
the filesystem cache, but we did not provide file extensions
used in vector search. This adds these extensions.

docs/reference/how-to/knn-search.asciidoc

@@ -48,6 +48,9 @@ good search performance.
 [discrete]
 include::search-speed.asciidoc[tag=warm-fs-cache]
 
+The following file extensions are used for the approximate kNN search:
+"vec" (for vector values), "vex" (for HNSW graph), "vem" (for metadata).
+
 [discrete]
 === Reduce vector dimensionality