net/url: document the package better

net/url: document the package better

Changes made:
* Adjust the documented form for a URL to make it more obvious what
happens when the scheme is missing.
* Remove references to Go1.5. We are sufficiently far along enough
that this distinction no longer matters.
* Remove the "Opaque" example which provides a hacky and misleading
use of the Opaque field. This workaround is no longer necessary
since RawPath was added in Go1.5 and the obvious approach just works:
	// The raw string "/%2f/" will be sent as expected.
	req, _ := http.NewRequest("GET", "https://example.com/%2f/")

Fixes #18824

Change-Id: Ie33d27222e06025ce8025f8a0f04b601aaee1513
Reviewed-on: https://go-review.googlesource.com/36127
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>