Fix javadoc warning in Strings for missing parameter description

Fixes a parameter in `Strings` that had a javadoc annotation but was missing the
description, causing warnings in the build.

server/src/main/java/org/elasticsearch/common/Strings.java

@@ -757,7 +757,7 @@ public class Strings {
 
     /**
      * Returns a string representation of the builder (only applicable for text based xcontent).
-     * @param xContentBuilder
+     * @param xContentBuilder builder containing an object to converted to a string
      */
     public static String toString(XContentBuilder xContentBuilder) {
         return BytesReference.bytes(xContentBuilder).utf8ToString();