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: https://github.com/nodejs/node/issues/6046
PR-URL: https://github.com/nodejs/node/pull/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>