[3.14] gh-141984: Reword and reorganize the first part of Atoms docs (GH-144117) (GH-144959)

(cherry picked from commit 112d8ac9724a53c5459a4f957941f5a3c97abf5d)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Blaise Pabon <blaise@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>