Add more information in PlayerHPChangeReason (#16024)

1) Add `node` and `node_pos` field for the `drown` field (analog to `node_damage` type)
2) Add `custom_type` field to allow a standardized way to report more details of the HP change
3) Make `builtin` report a `custom_type` for the HP changes caused by the `/kill` command and `core.do_item_eat`
4) Move the documentation of `PlayerHPChangeReason` to its own section
5) Fill in missing information of said documentation

src/server/player_sao.cpp

@@ -159,7 +159,8 @@ void PlayerSAO::step(float dtime, bool send_recommended)
 
 			// No more breath, damage player
 			if (m_breath == 0) {
-				PlayerHPChangeReason reason(PlayerHPChangeReason::DROWNING);
+				std::string nodename = c.name;
+				PlayerHPChangeReason reason(PlayerHPChangeReason::DROWNING, nodename, p);
 				setHP(m_hp - c.drowning, reason);
 			}
 		}

src/server/player_sao.h

@@ -250,7 +250,7 @@ struct PlayerHPChangeReason
 
 	// For PLAYER_PUNCH
 	ServerActiveObject *object = nullptr;
-	// For NODE_DAMAGE
+	// For NODE_DAMAGE and DROWNING
 	std::string node;
 	v3s16 node_pos;