This is an attempt to document the current state of signal handling.
It's not intended to describe the best way to handle signals.  Future
changes to signal handling should update these docs as appropriate.

update #9896.

Change-Id: I3c50af5cc641357b57dfe90ae1c7883a7e1ec059
Reviewed-on: https://go-review.googlesource.com/17877Reviewed-by: Russ Cox <rsc@golang.org>