Improved documentation of haveHttpHeader and deleteHeader

Minor wording improvements.

Question: `deleteHeader()` can only delete headers that were set by `haveHttpHeader()`? (Or other headers too?) So we could mention this and link to it?

Author: ThomasLandauer

src/Codeception/Module/REST.php

@@ -168,7 +168,7 @@ protected function getRunningClient()
     }
 
     /**
-     * Sets HTTP header valid for all next requests. Use `deleteHeader` to unset it
+     * Sets a HTTP header to be used for all subsequent requests. Use [`deleteHeader`](#deleteHeader) to unset it.
      *
      * ```php
      * <?php
@@ -188,8 +188,7 @@ public function haveHttpHeader($name, $value)
     }
 
     /**
-     * Deletes the header with the passed name.  Subsequent requests
-     * will not have the deleted header in its request.
+     * Deletes a HTTP header, so that subsequent requests will not send it anymore.
      *
      * Example:
      * ```php