[3.15] gh-141984: Reword docs on "enclosed" atom grammar (GH-148622) (GH-150552)

[3.15] gh-141984: Reword docs on "enclosed" atom grammar (GH-148622) (GH-150552)

Reorganize and reword the docs on atoms in parentheses, brackets and braces:
parenthesized groups, list/set/dict/tuple displays, and comprehensions.
(Generator expressions and yield atoms are left for later.)

In the spirit of better matching the underlying grammar, *comprehensions* are
covered separately from non-comprehension displays. Also, parenthesized forms
(with a single expression) and tuple displays are separated.
All sections are rewritten to start with simple cases and build up to the full
formal grammar.

(cherry picked from commit 55f25183263b9a52fa246817344cdb32d6c3d722)

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