Documentation for shared_parameters

The Python API includes config options for `shared_parameters` and `shared_params`, but these are not properly documented. The DyNet Python reference manual currently has uninformative descriptions like "Parameters: | shared_parameters (bool) – shared parameters".

I presume that this is a bool to allow parameters to be shared across processes. However, the documentation on multiprocessing (https://dynet.readthedocs.io/en/latest/multiprocessing.html) currently suggests that Python support is pending. What is the status of this? My impression from trying to use this is that setting `shared_parameters` to `True` allows the use of Python's built-in multiprocessing module with DyNet objects. I haven't encountered any problems so far, but I would appreciate hearing if there are any known limitations.

Start with the Python reference manual entries for `shared_parameters` and `shared_params`, then compare them with the multiprocessing documentation at dynet.readthedocs.io/en/latest/multiprocessing.html. Verify the options' behavior, Python multiprocessing support, and known limitations before replacing the uninformative descriptions with accurate guidance.

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