docs: add comments

Author: kenjis

system/HTTP/Message.php

@@ -61,6 +61,8 @@ public function getBody()
      *
      * @deprecated Use Message::headers() to make room for PSR-7
      *
+     * @TODO Incompatible return value with PSR-7
+     *
      * @codeCoverageIgnore
      */
     public function getHeaders(): array
@@ -76,6 +78,8 @@ public function getHeaders(): array
      *
      * @deprecated Use Message::header() to make room for PSR-7
      *
+     * @TODO Incompatible return value with PSR-7
+     *
      * @codeCoverageIgnore
      */
     public function getHeader(string $name)

system/HTTP/OutgoingRequestInterface.php

@@ -15,6 +15,8 @@
 
 /**
  * Representation of an outgoing, client-side request.
+ *
+ * Corresponds to Psr7\RequestInterface.
  */
 interface OutgoingRequestInterface extends MessageInterface
 {

system/HTTP/RequestInterface.php

@@ -13,6 +13,8 @@
 
 /**
  * Representation of an incoming, server-side HTTP request.
+ *
+ * Corresponds to Psr7\ServerRequestInterface.
  */
 interface RequestInterface extends OutgoingRequestInterface
 {