encoding: modernize Go documentation

Across all encoding packages, linkify declarations if possible.
In some cases, we convert a code block into a bulleted list,
which then further allows for more linkification.

Change-Id: I68fedf362615b34228bab5d4859b7d87d831c570
Reviewed-on: https://go-review.googlesource.com/c/go/+/524977


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>