runtime: update big mgc.go comment
runtime: update big mgc.go comment

The comment describing the overall GC algorithm at the top of mgc.go
has gotten woefully out-of-date (and was possibly never
correct/complete). Update it to reflect the current workings of the
GC and the set of phases that we now divide it into.

Change-Id: I02143c0ebefe9d4cd7753349dab8045f0973bf95
Reviewed-on: https://go-review.googlesource.com/34711
Reviewed-by: Rick Hudson <838bd060584c511f21c75c69ea04b7b120d86af0@golang.org>

derekmarcotte/go,josharian/go,cloudflare/go,CAFxX/go,golang/go,golang/go,ImJasonH/go,Cofyc/go,CAFxX/go,codestation/go,polyverse-security/go,cloudflare/go,vsekhar/elastic-go,linux-on-ibm-z/go,akutz/go,polyverse-security/go,josharian/go,CAFxX/go,ImJasonH/go,nawawi/go,derekmarcotte/go,polyverse-security/go,CAFxX/go,josharian/go,Cofyc/go,polyverse-security/go,josharian/go,golang/go,CAFxX/go,nawawi/go,golang/go,suncycheng/go,ImJasonH/go,vsekhar/elastic-go,ImJasonH/go,cloudflare/go,linux-on-ibm-z/go,CAFxX/go,derekmarcotte/go,Cofyc/go,derekmarcotte/go,derekmarcotte/go,derekmarcotte/go,CAFxX/go,nawawi/go,Cofyc/go,derekmarcotte/go,suncycheng/go,christopher-henderson/Go,polyverse-security/go,Cofyc/go,linux-on-ibm-z/go,golang/go,suncycheng/go,linux-on-ibm-z/go,Cofyc/go,akutz/go,suncycheng/go,polyverse-security/go,CAFxX/go,josharian/go,nawawi/go,akutz/go,akutz/go,cloudflare/go,polyverse-security/go,codestation/go,nawawi/go,vsekhar/elastic-go,akutz/go,vsekhar/elastic-go,akutz/go,cloudflare/go,codestation/go,ImJasonH/go,codestation/go,golang/go,nawawi/go,akutz/go,cloudflare/go,golang/go,derekmarcotte/go,suncycheng/go,vsekhar/elastic-go,suncycheng/go,josharian/go,vsekhar/elastic-go,codestation/go,ImJasonH/go,cloudflare/go,ImJasonH/go,linux-on-ibm-z/go,Cofyc/go,Cofyc/go,golang/go,linux-on-ibm-z/go,codestation/go,nawawi/go,akutz/go,ImJasonH/go,vsekhar/elastic-go,polyverse-security/go,suncycheng/go,suncycheng/go,codestation/go,cloudflare/go,codestation/go,josharian/go,vsekhar/elastic-go,nawawi/go,josharian/go,linux-on-ibm-z/go,linux-on-ibm-z/go