The full truth seems too complicated to write in this method's doc, so
I'm going with a simple half truth.

The full truth is that Fd returns the descriptor in blocking mode,
because that is historically how it worked, and existing programs
would be surprised if the descriptor is suddenly non-blocking. On Unix
systems whether a file is non-blocking or not is a property of the
underlying file description, not of a particular file descriptor, so
changing the returned descriptor to blocking mode also changes the
existing File to blocking mode. Blocking mode works fine, althoug I/O
operations now take up a thread. SetDeadline and friends rely on the
runtime poller, and the runtime poller only works if the descriptor is
non-blocking. So it's correct that calling Fd disables SetDeadline.
The other half of the truth is that if the program is willing to work
with a non-blocking descriptor, it could call
syscall.SetNonblock(descriptor, true) to change the descriptor, and
the original File, to non-blocking mode. At that point SetDeadline
would start working again. I tried to write that in a way that is
short and comprehensible but failed. Since deadlines mostly work on
pipes, and there isn't much reason to call Fd on a pipe, and few
people use SetDeadline, I decided to punt.

Fixes #22934

Change-Id: I2e49e036f0bcf71f5365193831696f9e4120527c
Reviewed-on: https://go-review.googlesource.com/81636Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>