Document Player_Action_Idle (#2266)

* document Player_Action_Idle

* Player_DecelerateToZero

* cleanup R_RUN_SPEED_LIMIT usages

* format

* typo

* reorder reg

* shake timer is used for fall damage

* comment on shake timer only being processed in one place

* format and edit comment

* shakeTimer -> fallDamageStunTimer

include/regs.h

@@ -48,6 +48,7 @@
 #define R_ENV_Z_FAR                              REG(13)
 #define R_ENV_FOG_NEAR                           REG(14)
 #define R_ENV_TIME_SPEED_OLD                     REG(15) // Most likely used during development. Unused in the final game.
+#define R_DECELERATE_RATE                        REG(43)
 #define R_RUN_SPEED_LIMIT                        REG(45)
 #define R_ENABLE_ARENA_DBG                       SREG(0)
 #define R_AUDIOMGR_DEBUG_LEVEL                   SREG(20)

include/z64player.h

@@ -881,6 +881,7 @@ typedef struct Player {
 
     /* 0x0850 */ union {
         s16 actionVar2;
+        s16 fallDamageStunTimer; // Player_Action_Idle: Prevents any movement and shakes model up and down quickly to indicate fall damage stun
         s16 bonked; // Player_Action_Roll: set to true after bonking into a wall or an actor
     } av2; // "Action Variable 2": context dependent variable that has different meanings depending on what action is currently running