I think of "sending" a signal as calling kill, but sigsend is involved
in handling a signal and, specifically delivering it to the internal
signal queue. The term "delivery" is already used in
signalWaitUntilIdle, so this CL also uses it in the documentation for
sigsend.

Change-Id: I86e171f247f525ece884a680bace616fa9a3c7bd
Reviewed-on: https://go-review.googlesource.com/81235Reviewed-by: Ian Lance Taylor <iant@golang.org>