[3.12] gh-107008: Document the curses module variables LINES and COLS (GH-107011) (GH-107057)

LINES and COLS referred in curses.update_lines_cols() documentations are
the module variables, not the environment variables.
(cherry picked from commit 26e08dfdd7ac1b3d567d30cd35e4898121580390)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>