Add capability to read table flag fields from Lua API

doc/lua_api.txt

@@ -410,6 +410,7 @@ All default ores are of the uniformly-distributed scatter type.
 
 Ore attributes
 -------------------
+See section Flag Specifier Format.
 Currently supported flags:  absheight
  - absheight
     Also produce this same ore between the height range of -height_max and -height_min.
@@ -451,6 +452,7 @@ Important note: Node aliases cannot be used for a raw schematic provided when re
 
 Schematic attributes
 ---------------------
+See section Flag Specifier Format.
 Currently supported flags:  place_center_x, place_center_y, place_center_z
  - place_center_x
     Placement of this decoration is centered along the X axis.
@@ -525,6 +527,26 @@ pointed_thing:
   {type="node", under=pos, above=pos}
   {type="object", ref=ObjectRef}
 
+Flag Specifier Format
+-----------------------
+Flags using the standardized flag specifier format can be specified in either of two ways, by string or table.
+The string format is a comma-delimited set of flag names; whitespace and unrecognized flag fields are ignored.
+Specifying a flag in the string sets the flag, and specifying a flag prefixed by the string "no" explicitly
+clears the flag from whatever the default may be.
+In addition to the standard string flag format, the schematic flags field can also be a table of flag names
+to boolean values representing whether or not the flag is set.  Additionally, if a field with the flag name
+prefixed with "no" is present, mapped to a boolean of any value, the specified flag is unset.
+
+e.g. A flag field of value
+  {place_center_x = true, place_center_y=false, place_center_z=true}
+is equivalent to
+  {place_center_x = true, noplace_center_y=true, place_center_z=true}
+which is equivalent to
+  "place_center_x, noplace_center_y, place_center_z"
+or even
+  "place_center_x, place_center_z"
+since, by default, no schematic attributes are set.
+
 Items
 ------
 Node (register_node):