runtime: fix documentation error about runtime.GC()
runtime: fix documentation error about runtime.GC()

runtime.GC no longer triggers a STW GC. This fixes the description of
GODEBUG=gctrace=1 so it doesn't claim otherwise.

Change-Id: Ibd34a55c5ae7b5eda5c2393b9a6674bdf1d51eb3
Reviewed-on: https://go-review.googlesource.com/45131
Reviewed-by: Rick Hudson <838bd060584c511f21c75c69ea04b7b120d86af0@golang.org>

golang/go,akutz/go,polyverse-security/go,CAFxX/go,golang/go,akutz/go,josharian/go,golang/go,josharian/go,nawawi/go,nawawi/go,vsekhar/elastic-go,CAFxX/go,codestation/go,akutz/go,Cofyc/go,cloudflare/go,vsekhar/elastic-go,CAFxX/go,polyverse-security/go,codestation/go,cloudflare/go,nawawi/go,akutz/go,vsekhar/elastic-go,josharian/go,codestation/go,cloudflare/go,Cofyc/go,golang/go,CAFxX/go,josharian/go,Cofyc/go,codestation/go,akutz/go,josharian/go,vsekhar/elastic-go,polyverse-security/go,golang/go,akutz/go,golang/go,cloudflare/go,cloudflare/go,Cofyc/go,polyverse-security/go,nawawi/go,vsekhar/elastic-go,CAFxX/go,CAFxX/go,akutz/go,polyverse-security/go,cloudflare/go,nawawi/go,Cofyc/go,josharian/go,golang/go,Cofyc/go,nawawi/go,polyverse-security/go,nawawi/go,christopher-henderson/Go,codestation/go,CAFxX/go,cloudflare/go,codestation/go,cloudflare/go,codestation/go,Cofyc/go,Cofyc/go,polyverse-security/go,akutz/go,josharian/go,vsekhar/elastic-go,josharian/go,golang/go,polyverse-security/go,nawawi/go,vsekhar/elastic-go,vsekhar/elastic-go,CAFxX/go,codestation/go