lua_api.md: Mapblock-related and misc improvements (#15972)

Co-authored-by: sfan5 <sfan5@live.de>
Co-authored-by: DS <ds.desour@proton.me>

doc/lua_api.md

@@ -1596,7 +1596,8 @@ There are a bunch of different looking node types.
 Node boxes
 ----------
 
-Node selection boxes are defined using "node boxes".
+Node selection boxes and collision boxes, and the appearance of the `nodebox`
+drawtype, are defined using "node boxes".
 
 A nodebox is defined as any of:
 
@@ -1681,8 +1682,8 @@ roughly 1x1x1 meters in size.
 
 A 'mapblock' (often abbreviated to 'block') is 16x16x16 nodes and is the
 fundamental region of a world that is stored in the world database, sent to
-clients and handled by many parts of the engine. This size is defined by the constant
+clients and handled by many parts of the engine. This size is available as the
-`core.MAP_BLOCKSIZE` (=16).
+constant `core.MAP_BLOCKSIZE` (=16).
 
 'mapblock' is preferred terminology to 'block' to help avoid confusion with
 'node', however 'block' often appears in the API.
@@ -1692,6 +1693,38 @@ A 'mapchunk' (sometimes abbreviated to 'chunk') is usually 5x5x5 mapblocks
 the map generator.
 The size in mapblocks has been chosen to optimize map generation.
 
+### Mapblock status
+
+A mapblock being "loaded" means that is in memory. These are the mapblocks that
+API functions like `core.get_node` or `core.set_node` can operate on. To reach
+this state, the mapblock must first go through the process of being "emerged".
+This means that it is loaded from disk, and/or, if it isn't yet generated,
+generated by the map generator.
+
+Mapblocks are loaded in a broad area around each player. They become "unloaded"
+again if no player is close enough. The engine commonly represents the contents
+of unloaded mapblocks as `"ignore"` nodes.
+
+A mapblock being "active" means that it is not only in memory, but also affected
+by world simulation:
+
+* Entities are active
+    * They are in memory as `ServerActiveObject`, exposed to Lua as `ObjectRef`
+    * They exist in Lua as luaentity tables
+* ABMs are executed
+* Node timers are executed
+
+Also, when a mapblock is "activated", LBMs are executed. Mapblocks are active
+in a smaller area around each player, and are "deactivated" again if no player
+is close enough.
+
+Related API functions:
+
+* `core.compare_block_status`
+* `core.forceload_block`
+* `core.load_area`
+* `core.emerge_area`
+
 Coordinates
 -----------
 
@@ -2032,10 +2065,14 @@ The following items are predefined and have special properties.
 
 * `"unknown"`: An item that represents every item which has not been registered
 * `"air"`: The node which appears everywhere where no other node is
-* `"ignore"`: Mapblocks which have not been yet generated consist of this node
+* `"ignore"`: Mapblocks that are not loaded are represented using this node.
-* `""`: The player's hand, which is in use whenever the player wields no item
+    * Also used for nodes that have not yet been set by the map generator.
-  * Its rage and tool capabilities are also used as an fallback for the wield item
+    * This is also what appears outside of the map boundary.
-  * It can be overridden to change those properties
+* `""`: The player's hand, which is in use whenever the player wields no item.
+    * Its range and tool capabilities are also used as a fallback for the wielded item.
+    * It can be overridden to change those properties:
+        * globally using `core.override_item`
+        * per-player using the special `"hand"` inventory list
 
 Amount and wear
 ---------------
@@ -7648,6 +7685,8 @@ Misc.
 
 * `core.forceload_block(pos[, transient[, limit]])`
     * forceloads the position `pos`.
+    * this means that the mapblock containing `pos` will always be kept in the
+      `"active"` state, regardless of nearby players or server settings.
     * returns `true` if area could be forceloaded
     * If `transient` is `false` or absent, the forceload will be persistent
       (saved between server runs). If `true`, the forceload will be transient
@@ -9442,6 +9481,10 @@ ABM (ActiveBlockModifier) definition
 
 Used by `core.register_abm`.
 
+An active block modifier (ABM) is used to define a function that is continously
+and randomly called for specific nodes (defined by `nodenames` and other conditions)
+in active mapblocks.
+
 ```lua
 {
     label = "Lava cooling",