docs: remove http streams from the list of writeables

The documentation lists HTTP server response and HTTP client request as writable streams. However, in fact, they are not, which confuses users. Example in this twitter thread: https://mobile.twitter.com/getify/status/1190456439996854273

I have checked other examples, they all extend stream.Writable, so they are fine to stay in the list.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines