LUCENE-10146: Add note that dot product is preferred over cosine (#400)

While VectorSimilarityFunction#COSINE is helpful when you need to preserve the
original vectors, it is significantly slower than DOT_PRODUCT. This commit adds
javadocs to COSINE explaining that dot product is the fastest option.

lucene/core/src/java/org/apache/lucene/index/VectorSimilarityFunction.java

@@ -56,7 +56,12 @@ public enum VectorSimilarityFunction {
     }
   },
 
-  /** Cosine similarity */
+  /**
+   * Cosine similarity. NOTE: the preferred way to perform cosine similarity is to normalize all
+   * vectors to unit length, and instead use {@link VectorSimilarityFunction#DOT_PRODUCT}. You
+   * should only use this function if you need to preserve the original vectors and cannot normalize
+   * them in advance.
+   */
   COSINE {
     @Override
     public float compare(float[] v1, float[] v2) {