Update documentation

[BV-WebDev]

Some documention fix and clarification:

• Fix HttpResponse.writeStatus lost doc.
• Rewrite HttpResponse onWritable, onAborted and onData docs.
• Update cork doc to specify that end* functions have to be corked too.
• Update HttpRequest.forEach doc to specify that keys and values are in lowercase.
• Fix TemplatedApp.filter doc invalid number type.

[jaime-ez]

Please merge this, I lost half a day on the writeStatus must be called first behaviour :)