runtime: document runtime.Frames better
runtime: document runtime.Frames better

In particular, this says that Frames.Function uniquely identifies a
function within a program. We depend on this in various places that
use runtime.Frames in std, but it wasn't actually written down.

Change-Id: Ie7ede348c17673e11ae513a094862b60c506abc5
Reviewed-on: https://go-review.googlesource.com/41610
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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