gh-138072: Small clarifications and phrasing improvements to asyncio HOWTO (#138073)

* - Small clarifications and phrasing improvements

* nit

* Apply suggestions from code review

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

* clarify event loops when multi threading.

* nit

* Update Doc/howto/a-conceptual-overview-of-asyncio.rst

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

* nit

* nit

* phrasing for threads & event loops.

* revert changes to event-loop/thread discussion.

* sentence case consistencty.

* slight re-arrange.

* Sentence case consistency.

* tweak language. non-negligible

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Co-authored-by: Peter Bierma <zintensitydev@gmail.com>