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

doc/lua_api.md

@@ -6226,26 +6226,14 @@ Call these functions only at load time!
       * Historically, the new HP value was clamped to [0, 65535] before
         calculating the HP change. This clamping has been removed as of
         version 5.10.0
-    * `reason`: a PlayerHPChangeReason table.
-        * The `type` field will have one of the following values:
-            * `set_hp`: A mod or the engine called `set_hp` without
-                        giving a type - use this for custom damage types.
-            * `punch`: Was punched. `reason.object` will hold the puncher, or nil if none.
-            * `fall`
-            * `node_damage`: `damage_per_second` from a neighboring node.
-                             `reason.node` will hold the node name or nil.
-                             `reason.node_pos` will hold the position of the node
-            * `drown`
-            * `respawn`
-        * Any of the above types may have additional fields from mods.
-        * `reason.from` will be `mod` or `engine`.
+    * `reason`: a `PlayerHPChangeReason` table.
     * `modifier`: when true, the function should return the actual `hp_change`.
        Note: modifiers only get a temporary `hp_change` that can be modified by later modifiers.
        Modifiers can return true as a second argument to stop the execution of further functions.
        Non-modifiers receive the final HP change calculated by the modifiers.
 * `core.register_on_dieplayer(function(ObjectRef, reason))`
     * Called when a player dies
-    * `reason`: a PlayerHPChangeReason table, see register_on_player_hpchange
+    * `reason`: a `PlayerHPChangeReason` table
     * For customizing the death screen, see `core.show_death_screen`.
 * `core.register_on_respawnplayer(function(ObjectRef))`
     * Called when player is to be respawned
@@ -6986,7 +6974,7 @@ Formspec functions
         * `"VAL"`: not changed
 * `core.show_death_screen(player, reason)`
     * Called when the death screen should be shown.
-    * `player` is an ObjectRef, `reason` is a PlayerHPChangeReason table or nil.
+    * `player` is an ObjectRef, `reason` is a `PlayerHPChangeReason` table or nil.
     * By default, this shows a simple formspec with the option to respawn.
       Respawning is done via `ObjectRef:respawn`.
     * You can override this to show a custom death screen.
@@ -8495,7 +8483,7 @@ child will follow movement and rotation of that bone.
     * note: this is called `right_click` for historical reasons only
 * `get_hp()`: returns number of health points
 * `set_hp(hp, reason)`: set number of health points
-    * See reason in register_on_player_hpchange
+    * reason: A `PlayerHPChangeReason` table (optional)
     * Is limited to the range of 0 ... 65535 (2^16 - 1)
     * For players: HP are also limited by `hp_max` specified in object properties
 * `get_inventory()`: returns an `InvRef` for players, otherwise returns `nil`
@@ -11314,6 +11302,40 @@ See [Decoration types](#decoration-types). Used by `core.register_decoration`.
 }
 ```
 
+`PlayerHPChangeReason` table definition
+---------------------------------------
+
+The `PlayerHPChangeReason` table specifies a reason for player health changes.
+
+* The `type` field is for providing one of the possible damage types
+  supported natively by the engine. It will have one of the following values:
+    * `set_hp`: A mod, builtin or the engine called `set_hp`, either
+       without giving a damage type, or by setting `set_hp`
+       as damage type explicitly
+    * `punch`: Was punched. `reason.object` will hold the puncher, or nil if none.
+    * `fall`: Fall damage.
+    * `node_damage`: `damage_per_second` from a neighboring node.
+                     `reason.node` will hold the node name or nil.
+                     `reason.node_pos` will hold the position of the node
+    * `drown`: Drowning damage from a node with the `drowning` field set.
+               `reason.node` and `reason.node_pos` are same as for `node_damage`
+    * `respawn`: HP restored by respawning.
+* The `custom_type` field may optionally be used to provide a reason that is not
+    supported by the engine, as a string. It will be ignored by the engine,
+    but it can be used to communicate to other mods about custom damage types.
+    If provided, it must be a string. It's recommended to follow the
+    `modname:reason` naming convention.
+    These custom types exist by default:
+    * `__builtin:item_eat`: HP change caused by `core.do_item_eat`
+    * `__builtin:kill_command`: `/kill` command
+* The `from` field denotes the origin of the HP change:
+    * `engine`: Engine
+    * `mod`: Mod or builtin
+* Mods may add additional fields
+
+Note: The `from` is ignored by `ObjectRef:set_hp`, the engine will always
+set it to `mod`.
+
 Chat command definition
 -----------------------