doc: document encoding/xml change in CL 203417
doc: document encoding/xml change in CL 203417

Change-Id: Ibc0228f166f449ec28d813f33bdb550fe7ba2b3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/236739
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/go,golang/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,akutz/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,akutz/go,nawawi/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,akutz/go