Document hpchange callback ordering thing (#3981)

Document hpchange callback ordering thing (#3981)

Document hpchange callback ordering thing

Callbacks registered by register_on_player_hpchange are ordered so that non-modifiers are called after modifiers are called. Credit to @TeTpaAka who mentioned this previously-undocumented feature in #3799.

See also commit

aa13baa30a45b0f834c23bd5c0407895eb8ec0ee "Add minetest.register_on_player_hpchange"