runtime: improve sigsend documentation
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/81235
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,cloudflare/go,akutz/go,josharian/go,akutz/go,CAFxX/go,Cofyc/go,josharian/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,nawawi/go,golang/go,golang/go,codestation/go,golang/go,Cofyc/go,CAFxX/go,josharian/go,akutz/go,codestation/go,codestation/go,akutz/go,cloudflare/go,cloudflare/go,Cofyc/go,golang/go,codestation/go,golang/go,golang/go,Cofyc/go,CAFxX/go,codestation/go,josharian/go,CAFxX/go,codestation/go,Cofyc/go,akutz/go,golang/go,cloudflare/go,josharian/go,cloudflare/go,Cofyc/go,josharian/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,christopher-henderson/Go,codestation/go,josharian/go,josharian/go,nawawi/go,codestation/go,CAFxX/go,cloudflare/go,cloudflare/go,Cofyc/go,nawawi/go,Cofyc/go,akutz/go,nawawi/go,CAFxX/go