[release-branch.go1.13] encoding/json: revert Compact HTML escaping documentation

This partly reverts CL 173417 as it incorrectly documented that Compact
performed HTML escaping and the output was safe to embed inside HTML
<script> tags. This has never been true.

Although Compact does escape U+2028 and U+2029, it doesn't escape <, >
or &. Compact is thus only performing a subset of HTML escaping and it's
output is not safe to embed inside HTML <script> tags.

A more complete fix would be for Compact to either never perform any
HTML escaping, as it was prior to CL 10883045, or to actually perform
the same HTML escaping as HTMLEscape. Neither change is likely safe
enough for go1.13.

Fixes #34006
Updates #30357

Change-Id: I912f0fe9611097d988048b28228c4a5b985080ba
GitHub-Last-Rev: aebababc9233c5705785b225377e80096d4bb8c4
GitHub-Pull-Request: golang/go#33427
Reviewed-on: https://go-review.googlesource.com/c/go/+/188717


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 79669dc7)
Reviewed-on: https://go-review.googlesource.com/c/go/+/192747


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>