Improvements to Origin docs

[brson]

The docs here are a little sparse. I wanted to know what an origin is. I think the docs for Origin itself should explain in prose something like "origin is a combination of scheme/host/port, but some URLs have no origin in these conditions: ...".

The description for Origin::Opaque says "a globally unique identifier", which is not really what I think of as "opaque". Could use a better explanation of the purpose of making Opaque a guid. #servo will know.

[SimonSapin]

An "origin" is the basis of web browsers’s same-origin policy, and Cross-Origin Resource Sharing (CORS) a.k.a. HTTP access control:

https://developer.mozilla.org/en-US/docs/Web/Security/Same-origin_policy
https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS

Roughly, it represents “Are these two URLs from the same site?”