doc/go1.16: add encoding/json note for tag change
doc/go1.16: add encoding/json note for tag change

For #40700
Fixes #42898

Change-Id: I652657ff8d6cce20bf868f0b1101d723d3f704d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/274614
Trust: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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