net/url: improve URL docs

net/url: improve URL docs

The Raw fields are confusing and easy to use by mistake. Adds more
context in comments to these fields.

Also the current docs (and the names of these fields) of these
boolean fields are not obvious that parser might produce them,
so clarify that

Change-Id: I6a6a69644834c3ccbf657147f771930b6875f721
Reviewed-on: https://go-review.googlesource.com/c/go/+/706515
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>