gh-144067: Document terminal leak when initscr() follows setupterm() (GH-152624)

gh-144067: Document terminal leak when initscr() follows setupterm() (GH-152624)

The curses library keeps a single current terminal: calling initscr()
or newterm() after setupterm() abandons the terminal that setupterm()
allocated without freeing it.  This cannot be fixed safely in code
(freeing the terminal would break the capsule API and pending terminfo
queries), so document the behavior, with a cross-reference from
initscr() and newterm().

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>