protocol: disambiguate key codes in wl_keyboard.key

Explain that wl_keyboard.key yields platform-specific key codes.
Some compositors use Linux key codes (defined in the
linux/input-event-codes.h header file, e.g. KEY_ESC), however
clients should not assume that this is always the case. The only
reliable way for clients to interpret key codes is to feed them
into a keyboard mapping.

Signed-off-by: Simon Ser <contact@emersion.fr>