doc/go1.16: document encoding/json changes for Go 1.16

[dmitshur]

As of filing this issue, the Go 1.16 draft release notes contained the following HTML:

encoding/json

The error message for SyntaxError now begins with "json: ", matching the other errors in the package.

TODO: https://golang.org/cl/234818: allow semicolon in field key / struct tag

---

Per golang.org/s/release, it's a release blocker for Go 1.16 Beta 1 to have a complete draft of the eventual release notes, and so the TODO needs to be resolved.

A sequence of steps to resolve this issue may be:

1. Come up with a draft release note entry in this issue, then move the issue to NeedsFix state.
2. Send a CL with "doc/go1.16:" prefix and include "For doc: write Go 1.16 release notes #40700. Fixes #{this issue}." in the body.

[dsnet]

We'll probably roll back this change. See #42675.

[gopherbot]

Change https://golang.org/cl/274614 mentions this issue: doc/go1.16: add encoding/json note for tag change