[3.12] GH-108202: Document ``calendar``'s command-line interface (GH-109020) (#109030)

GH-108202: Document ``calendar``'s command-line interface (GH-109020)
(cherry picked from commit f0f96a9f40762499811681d405b6f922b6ed7a55)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>