Semantic and documentation of Interpreter.prepare_main()

Interpreter.prepare_main() is documented as:

   .. method:: prepare_main(ns=None, **kwargs)
      
      Bind objects in the interpreter's :mod:`!__main__` module.
   
      Some objects are actually shared and some are copied efficiently,
      but most are copied via :mod:`pickle`.  See :ref:`interp-object-sharing`.

But it is not clear what ns is, and how kwargs is related. A module and a class are nemspaces, so can you pass them? Or the SimpleNamespace instance? No. Looking at the code, ns can only be None, a mapping or an iterable of pairs. Is it what was intended? Than it should be documented.

But accepting an iterable of pairs may be not intended. This can hardly be called a namespace. Then the code should be changed from

ns = dict(ns, **kwargs) if ns is not None else kwargs

to

ns = {**ns, **kwargs} if ns is not None else kwargs

And was it necessary to accept None? This is error-prone, as you can accidentally get None instead of a valid value (missing return or missing initialization). {} could be used as a default value, it is fine if it is not mutated. If exclude None, it can be simply written as:

ns = {**ns, **kwargs}

And I suggest to to change the name of the parameter, as it is not a general namespace. "dict" or "mapping" would better reflect its type. "updates" or "changes" could also be used, but then we should also rename "kwargs" to "updates2" or "changes2". Since this is positional-only parameter, changing the name does not break user code.

cc @ericsnowcurrently

Start with the Interpreter.prepare_main() documentation and its implementation, especially how ns and kwargs are combined. Determine the intended accepted input types and whether the parameter names or behavior should change, then update the documentation or related code and add or adjust the relevant tests so the API semantics are explicit.