Lua_api.txt: Document blockpos, coordinate conversion, map terminology (#8376)

Also fix and improve LVM 'calc_lighting()' warning.

doc/lua_api.txt

@@ -1171,6 +1171,64 @@ A box of a regular node would look like:
 
 
 
+Map terminology and coordinates
+===============================
+
+Nodes, mapblocks, mapchunks
+---------------------------
+
+A 'node' is the fundamental cubic unit of a world and appears to a player as
+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.
+'mapblock' is preferred terminology to 'block' to help avoid confusion with
+'node', however 'block' often appears in the API.
+
+A 'mapchunk' (sometimes abbreviated to 'chunk') is usually 5x5x5 mapblocks
+(80x80x80 nodes) and is the volume of world generated in one operation by
+the map generator.
+The size in mapblocks has been chosen to optimise map generation.
+
+Coordinates
+-----------
+
+### Orientation of axes
+
+For node and mapblock coordinates, +X is East, +Y is up, +Z is North.
+
+### Node coordinates
+
+Almost all positions used in the API use node coordinates.
+
+### Mapblock coordinates
+
+Occasionally the API uses 'blockpos' which refers to mapblock coordinates that
+specify a particular mapblock.
+For example blockpos (0,0,0) specifies the mapblock that extends from
+node position (0,0,0) to node position (15,15,15).
+
+#### Converting node position to the containing blockpos
+
+To calculate the blockpos of the mapblock that contains the node at 'nodepos',
+for each axis:
+
+* blockpos = math.floor(nodepos / 16)
+
+#### Converting blockpos to min/max node positions
+
+To calculate the min/max node positions contained in the mapblock at 'blockpos',
+for each axis:
+
+* Minimum:
+  nodepos = blockpos * 16
+* Maximum:
+  nodepos = blockpos * 16 + 15
+
+
+
+
 HUD
 ===
 
@@ -3193,10 +3251,8 @@ Methods
     * To be used only by a `VoxelManip` object from
       `minetest.get_mapgen_object`.
     * (`p1`, `p2`) is the area in which lighting is set, defaults to the whole
-      area if left out or nil.
+      area if left out or nil. For almost all uses these should be left out
-    * Setting `p1`, `p2` to `emin`, `emax` of the mapgen object voxelmanip (the
+      or nil to use the default.
-      entire volume of the voxelmanip: the mapchunk plus a 1 mapblock thick
-      shell around it) will cause an error.
     * `propagate_shadow` is an optional boolean deciding whether shadows in a
       generated mapchunk above are propagated down into the mapchunk, defaults
       to `true` if left out.