libretro.h: warn that input polling must use base device IDs

The @param device line for retro_input_state_t documents only that the
argument is masked with RETRO_DEVICE_MASK, which in isolation reads as a
promise that subclass IDs are handled at the poll boundary. The masking is
a frontend-side convenience (RetroArch strips the subclass bits; not all
frontends do), and per the RETRO_DEVICE_SUBCLASS note, polling with a
subclass ID is reserved for future definition. Add a @warning cross-
referencing that reservation so core authors do not rely on the mask.

Author: LibretroAdmin

libretro-common/include/libretro.h

@@ -7716,6 +7716,9 @@ typedef void (RETRO_CALLCONV *retro_input_poll_t)(void);
  *
  * @param port Which player 'port' to query.
  * @param device Which device to query for. Will be masked with \c RETRO_DEVICE_MASK.
+ * @warning Poll with a base device ID only; passing an ID created via
+ * \c RETRO_DEVICE_SUBCLASS is reserved for future definition, and the masking
+ * noted above is a frontend convenience that must not be relied upon.
  * @param index The input index to retrieve.
  * The exact semantics depend on the device type given in \c device.
  * @param id The ID of which value to query, like \c RETRO_DEVICE_ID_JOYPAD_B.