[3.13] Pedantic rewording of why relative importing doesn't work in main modules (GH-136846) (#136941)

Pedantic rewording of why relative importing doesn't work in main modules (GH-136846)

Pedantically reword the section about relative imports and main modules.
(cherry picked from commit 4b68289ca6954b8d135e2ee2344e67fae38239fd)

Co-authored-by: Josh Cannon <joshdcannon@gmail.com>