Clarify client constructor host option

[danielmewes]

Feedback from @ejsuncy rethinkdb/horizon#725:

The client Horizon model has a constructor that takes an object of option keys and values, one of which is host. It is not apparent from the docs here that the host option is a string that includes the port number if applicable. Perhaps providing an example or two there such as 'host':'localhost:8181' would be more helpful, as I had to dig in to the source to find the constructor of the Horizon object.

I bring this up because I'm developing my web app and using BrowserSync, which serves the static files on port 5555, for example. I'm also using horizon on 8181, and I need to override the default port that Horizon client tries to connect to (in this case, the default was port 5555).

Thanks!