Fix GenServer.cast/2 docs

Removes ambiguity about what occurs when an atom is used to reference a
GenServer.

Clarifies comment about another process being used to send a cast in
OTP. Another process is only used if there is no connection the node and
only casts to other nodes are not guaranteed to arrive in order.
Merge pull request #2897 from fishcakez/server_cast_typo

Fix GenServer.cast/2 docs
Merge branch 'master' of https://github.com/elixir-lang/elixir