[dev.typeparams] cmd/compile: add documentation for unified IR pipeline

While at it, also rename "useUnifiedIR" to "unified", to be consistent
with "-d=unified" and "GOEXPERIMENT=unified".

Change-Id: I48ffdb4b36368343893b74f174608f5f59278249
Reviewed-on: https://go-review.googlesource.com/c/go/+/328989


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>