asyncio: fix environment variables not appearing in docs (#4261)

* asyncio: fix environment variables not in docs

Fix the docstrings in environment_variables.py so they are correctly
picked up by Sphinx autodoc for Read the Docs.

Previously the docstrings were placed above the variable assignments,
which Sphinx cannot parse. Moved them below and added the
required '.. envvar::' directives, consistent with how other
packages (e.g. opentelemetry-instrumentation) document their
environment variables.

Fixes #4256

* Fix typo from 'determines' to 'determine'

* Apply suggestion from @xrmx

* asyncio: move environment_variables docs to __init__ module docstring for sphinx

* asyncio: keep envvar docs in environment_variables.py with module-level docstring

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>