Fix exception descriptions

Author: sagikazarmark

src/Exception/HttpException.php

@@ -6,7 +6,9 @@
 use Psr\Http\Message\ResponseInterface;
 
 /**
- * Thrown when both a request and a response are available
+ * Thrown when a response was received but has an error status code.
+ *
+ * This exception always provides the request and response objects.
  *
  * @author Márk Sági-Kazár <mark.sagikazar@gmail.com>
  */

src/Exception/RequestException.php

@@ -5,7 +5,9 @@
 use Psr\Http\Message\RequestInterface;
 
 /**
- * Thrown when a request is available
+ * Base exception for when a request failed.
+ *
+ * This can be because of network problems or the status code of the response.
  *
  * @author Márk Sági-Kazár <mark.sagikazar@gmail.com>
  */

src/Exception/TransferException.php

@@ -3,7 +3,7 @@
 namespace Http\Client\Exception;
 
 /**
- * Thrown when an error related to sending/receiving the request/response occurs
+ * Base exception for all php-http exceptions.
  *
  * @author Márk Sági-Kazár <mark.sagikazar@gmail.com>
  */