net/http: change TimeoutHandler's docs to match its new interfaces

As of Go 1.13rc1, TimeoutHandler supports the Flusher and Pusher interfaces and
this change corrects its documentation to say that.

Fixes #33769
Updates #29193

Change-Id: Ia0523f7f2e3dc1f8f0b68950b85a7bf81c4abe60
GitHub-Last-Rev: 5310d2c9608a1af2d3030a9573e920906c76744e
GitHub-Pull-Request: golang/go#33770
Reviewed-on: https://go-review.googlesource.com/c/go/+/191237


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>