doc: mention handling of moved GOROOT in 1.9 release notes
doc: mention handling of moved GOROOT in 1.9 release notes

Updates #20587

Change-Id: Ia131b9a4dc4986950d9ecbfcbd6b026ade234fc0
Reviewed-on: https://go-review.googlesource.com/53370
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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