diff --git a/src/Composer/Util/HttpDownloader.php b/src/Composer/Util/HttpDownloader.php
index 9bd1cb6f0..8f3d606d8 100644
--- a/src/Composer/Util/HttpDownloader.php
+++ b/src/Composer/Util/HttpDownloader.php
@@ -73,6 +73,16 @@ class HttpDownloader
         $this->rfs = new RemoteFilesystem($io, $config, $options, $disableTls);
     }
 
+    /**
+     * Download a file synchronously
+     *
+     * @param  string   $url     URL to download
+     * @param  array    $options Stream context options e.g. https://www.php.net/manual/en/context.http.php
+     *                           although not all options are supported when using the default curl downloader
+     * @return Response
+     *
+     * @throws TransportException
+     */
     public function get($url, $options = array())
     {
         list($job) = $this->addJob(array('url' => $url, 'options' => $options, 'copyTo' => false), true);
@@ -101,6 +111,16 @@ class HttpDownloader
         return $response;
     }
 
+    /**
+     * Create an async download operation
+     *
+     * @param  string   $url     URL to download
+     * @param  array    $options Stream context options e.g. https://www.php.net/manual/en/context.http.php
+     *                           although not all options are supported when using the default curl downloader
+     * @return Promise
+     *
+     * @throws TransportException
+     */
     public function add($url, $options = array())
     {
         list(, $promise) = $this->addJob(array('url' => $url, 'options' => $options, 'copyTo' => false));
@@ -108,6 +128,17 @@ class HttpDownloader
         return $promise;
     }
 
+    /**
+     * Copy a file synchronously
+     *
+     * @param  string   $url     URL to download
+     * @param  string   $to      Path to copy to
+     * @param  array    $options Stream context options e.g. https://www.php.net/manual/en/context.http.php
+     *                           although not all options are supported when using the default curl downloader
+     * @return Response
+     *
+     * @throws TransportException
+     */
     public function copy($url, $to, $options = array())
     {
         list($job) = $this->addJob(array('url' => $url, 'options' => $options, 'copyTo' => $to), true);
@@ -116,6 +147,17 @@ class HttpDownloader
         return $this->getResponse($job['id']);
     }
 
+    /**
+     * Create an async copy operation
+     *
+     * @param  string   $url     URL to download
+     * @param  string   $to      Path to copy to
+     * @param  array    $options Stream context options e.g. https://www.php.net/manual/en/context.http.php
+     *                           although not all options are supported when using the default curl downloader
+     * @return Promise
+     *
+     * @throws TransportException
+     */
     public function addCopy($url, $to, $options = array())
     {
         list(, $promise) = $this->addJob(array('url' => $url, 'options' => $options, 'copyTo' => $to));
@@ -287,6 +329,11 @@ class HttpDownloader
         $this->runningJobs--;
     }
 
+    /**
+     * Wait for current async download jobs to complete
+     *
+     * @param int|null $index For internal use only, the job id
+     */
     public function wait($index = null)
     {
         while (true) {
@@ -309,7 +356,8 @@ class HttpDownloader
     /**
      * @internal
      *
-     * @return int number of active (queued or started) jobs
+     * @param  int|null $index For internal use only, the job id
+     * @return int      number of active (queued or started) jobs
      */
     public function countActiveJobs($index = null)
     {
@@ -362,6 +410,9 @@ class HttpDownloader
         return $resp;
     }
 
+    /**
+     * @internal
+     */
     public static function outputWarnings(IOInterface $io, $url, $data)
     {
         foreach (array('warning', 'info') as $type) {
@@ -382,6 +433,9 @@ class HttpDownloader
         }
     }
 
+    /**
+     * @internal
+     */
     public static function getExceptionHints(\Exception $e)
     {
         if (!$e instanceof TransportException) {
@@ -427,6 +481,9 @@ class HttpDownloader
         return true;
     }
 
+    /**
+     * @internal
+     */
     public static function isCurlEnabled()
     {
         return \extension_loaded('curl') && \function_exists('curl_multi_exec') && \function_exists('curl_multi_init');