runtime: improve sigsend documentation

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>

src/runtime/sigqueue.go

@@ -61,8 +61,9 @@ const (
 	sigSending
 )
 
-// Called from sighandler to send a signal back out of the signal handling thread.
-// Reports whether the signal was sent. If not, the caller typically crashes the program.
+// sigsend delivers a signal from sighandler to the internal signal delivery queue.
+// It reports whether the signal was sent. If not, the caller typically crashes the program.
+// It runs from the signal handler, so it's limited in what it can do.
 func sigsend(s uint32) bool {
 	bit := uint32(1) << uint(s&31)
 	if !sig.inuse || s >= uint32(32*len(sig.wanted)) {