reflect: document how DeepEqual handles cycles
reflect: document how DeepEqual handles cycles

Fixes #20428

Change-Id: Ia450e615728efd4ccb6e42117b547cac162f13a3
Reviewed-on: https://go-review.googlesource.com/52931
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

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