Document "observer" vs. "subscriber"

[DavidMGross]

New Rx users are confused about the distinction between "observer" and "subscriber" and need to know when to use one or the other. The documentation in general may use those terms casually and imprecisely, also.

See ReactiveX/RxJava#3201

[DavidMGross]

Ping @artem-zinnatullin as a possible contributor to this one.

See also: http://stackoverflow.com/questions/27664221/what-is-the-difference-between-an-observer-and-a-subscriber