doc/go1.13: document net/http.Transport.Request.Body unfurling

Documents the work from:
* CL 163599
* CL 163737

which now uses the Request.Body's io.ReaderFrom implementation,
if available, and permits system level optimizations such as
"sendfile" to be used to transmit/upload the Body, which greatly
speeds up file uploads.

Updates #33396

Change-Id: I7b8315c4b3e57ad47bb9be2b0c838857875d4bd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/188457


Reviewed-by: Andrew Bonventre <andybons@golang.org>