Update authentication method docs

[marcfrederick]

Attempt to resolve #322, by clarifying the authentication section of the documentation.

[AdamWill]

I kinda preferred how the existing file has OAuth first. We really want to encourage people to use OAuth for what's probably the most common use case (connecting to wikimedia wikis). In my experience you should always put the thing people are usually meant to do first, then other stuff comes afterwards.

[marcfrederick]

In my experience you should always put the thing people are usually meant to do first, then other stuff comes afterwards.

Good point, I've switched the sections back to the original order.

[AdamWill]

Should we also document clientlogin()?

[AdamWill]

Well, this is clearly an improvement anyway, so let's go with it. I guess I'll write a follow-up to document clientlogin.