Improve Connection.open/2 typespecs

[britto]

To better document all the 3 possible argument combinations and make them stricter:

• uri, name
• options, name
• uri, options

The previous spec covered invalid combinations, such as (keyword, keyword) and made it a bit hard to understand the possibilities without diving into the source code.

Output preview

[britto]

Oh, just saw #167. Looks like we're favoring passing :name through options instead, so we'd better not advertise the older (soon to be deprecated?) format.