doc: clarifies http.serverResponse implementation

Since http.serverResponse does not inherit from Stream.writable
it does not pass the test `serverResponse instanceof stream.Writable`.
This commit clarifies that serverResponse does not inherit from
stream.Writable and therefore should not be expected to pass the above
test

Fixes: nodejs#6046
PR-URL: nodejs#6072
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/http.md

@@ -638,8 +638,8 @@ connections.
 This object is created internally by a HTTP server--not by the user. It is
 passed as the second parameter to the `'request'` event.
 
-The response implements the [Writable Stream][] interface. This is an
-[`EventEmitter`][] with the following events:
+The response implements, but does not inherit from, the [Writable Stream][]
+interface. This is an [`EventEmitter`][] with the following events:
 
 ### Event: 'close'