Add a comment clarifying standards-compliant getch() behavior

After a lengthy forum debate involving an alternate and streamlined implementation of getch(), I discovered that getch() IS in fact required to call refresh() at times. It's unclear whether InvalidateRect (a Windows-specific function) is required as well, but a screen refresh is the desired outcome.

Leaving a comment to that effect, so that future programmers aren't lead on another wild goose chase when they notice that getch is refreshing the window without an obvious reason.