doc: Add explanation that -alt-grab and -ctrl-grab affect special keys

doc: Add explanation that -alt-grab and -ctrl-grab affect special keys

Phillip Merensky reported that the special keys (e.g. Ctrl-Alt-f for full
screen) did not work correctly if -alt-grab is used.

BUG: 696530

Review of ui/sdl.c:sdl_refresh indicates that this is the intended behaviour,
so we should update the documentation to match the actual behaviour, as
suggested by Phillip in the bug report.

Signed-off-by: Brad Hards <bradh@frogmouth.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>