Lua API documentation: Various fixes (#8914)

Lua API documentation: Various fixes (#8914)

Remove unusable 'minetest.setting_*' from client_lua_api.txt
lua_api.txt:
- Define the 'mod.conf' format
- More precise 'settingtypes.txt' format reference
- Document special sound files 'player_*damage'
- Group, sort and add 'not_in_creative_inventory' special group
- Define the 'Settings' format
- Warning about incorrect byte saving in 'StorageRef'
- Note about non-persistent player definitions fields
- Better 'leveldiff' and 'level' group description