Add warning for mix phx.server when sever: false in Endpoint

[gilbertbw]

Add a warning if you run mix phx.sever but have set sever: false in Endpoint. Without this change you get no output when running mix phx.server is this scenario.

Also document this.

[SteffenDE]

Since there might be multiple endpoints, let's move the warning to the init function where we also have the module:

    server? = server?(conf)

    if not server? and Application.get_env(:phoenix, :serve_endpoints, false) do
      # warn if mix phx.server is used but server is explicitly set to false
      Logger.warning(
        ":server is set to :false in your endpoint configuration for #{inspect(mod)}, so the server will not be started"
      )
    end

Wdyt?