Add doc for param.

Author: Mike Ng

android-sdk/src/main/java/com/optimizely/ab/android/sdk/OptimizelyManager.java

@@ -322,6 +322,7 @@ public boolean isDatafileCached(Context context) {
 
     /**
      * Returns the URL of the versioned datafile that this SDK expects to use
+     * @param projectId The id of the project for which we are getting the datafile
      * @return the CDN location of the datafile
      */
     public static @NonNull String getDatafileUrl(String projectId) {