bpo-34206: Improve docs and test coverage for pre-init functions (#8023)

- move the Py_Main documentation from the very high level API section
  to the initialization and finalization section
- make it clear that it encapsulates a full Py_Initialize/Finalize
  cycle of its own
- point out that exactly which settings will be read and applied
  correctly when Py_Main is called after a separate runtime
  initialization call is version dependent
- be explicit that Py_IsInitialized can be called prior to
  initialization
- actually test that Py_IsInitialized can be called prior to
  initialization
- flush stdout in the embedding tests that run code so it appears
  in the expected order when running with "-vv"
- make "-vv" on the subinterpreter embedding tests less spammy

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Co-authored-by: Carol Willing <carolcode@willingconsulting.com>