Supervisor.which_children has incorrect `@spec`

[boydm]

Current behavior

The documentation indicates that which_children returns a list of modules. This is confirmed in the code. It returns a list of modules.

The @spec for the function indicates it returns a single module or :dynamic.

Thus, the following code matches and works at runtime, but produces a dialyzer error.

[{_, pid, :supervisor, [DynamicSupervisor]}] = Supervisor.which_children(supervisor_pid)

Expected behavior

The @spec for which_children should be:

which_children(supervisor()) :: [
  {term() | :undefined, child() | :restarting, :worker | :supervisor,
   [module()] | :dynamic}
]

The change is that the module() toward the end is in a list.

Note: the :dynamic might need to be in the list too. Not sure without digging in deeper.