Rename and document HITMARKER_x100000

This marker makes the target unable to Counter or Mirror Coat the damage back to an attacker, however the game does not have a scenario where this is applicable since all its uses are in end-turn events. It's used for all passive damage, where the damage dealt isn't coming from an attacker.
Alternate name: HITMARKER_NO_ATTACKER

include/constants/battle.h

@@ -172,7 +172,7 @@
 #define HITMARKER_IGNORE_UNDERGROUND    0x00020000
 #define HITMARKER_IGNORE_UNDERWATER     0x00040000
 #define HITMARKER_UNABLE_TO_USE_MOVE    0x00080000
-#define HITMARKER_x100000               0x00100000
+#define HITMARKER_PASSIVE_DAMAGE        0x00100000
 #define HITMARKER_x200000               0x00200000
 #define HITMARKER_PLAYER_FAINTED        0x00400000
 #define HITMARKER_x800000               0x00800000