Add nil as documented valid option for GenServer :name (#15187)

lib/elixir/lib/gen_server.ex

@@ -232,6 +232,8 @@ defmodule GenServer do
   a name on start via the `:name` option. Registered names are also
   automatically cleaned up on termination. The supported values are:
 
+    * `nil` (default) - the GenServer is not registered with a name.
+
     * an atom - the GenServer is registered locally (to the current node)
       with the given name using `Process.register/2`.
 
@@ -858,7 +860,7 @@ defmodule GenServer do
   @type on_start :: {:ok, pid} | :ignore | {:error, {:already_started, pid} | term}
 
   @typedoc "The GenServer name"
-  @type name :: atom | {:global, term} | {:via, module, term}
+  @type name :: nil | atom | {:global, term} | {:via, module, term}
 
   @typedoc "Options used by the `start*` functions"
   @type options :: [option]