Looks like a bit of hasty copying and pasting from s_item.cpp.

Remove flagmask field from set_mapgen_params table
Add small bits of needed documentation

MapgenParams is no longer a polymorphic class, eliminating the need for messy and bug-prone reallocations.
Separation between the common and mapgen-specific parameters is now strongly defined.
Mapgen parameters objects are now properly encapsulated within the proper subsystems.

Use tables for adding particles, deprecate former way.

separate particles(pawner) definition, add default values, work with no
arguments

Lua does not guarantee that the indexes of a table will be in numerical order.

Indexes for empty slots shall not be skipped.