gh-124832: Add a note to indicate that `datetime.now` may return the same instant (#124834)

* Update datetime.rst

* Update datetime.rst

replace warning with note

* Update Doc/library/datetime.rst

Co-authored-by: Victor Stinner <vstinner@python.org>

* Update Doc/library/datetime.rst

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

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Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>