Improve code

[wuniversales]

Change:

local S = minetest.get_translator("pontoons")
with
local S = minetest.get_translator(minetest.get_current_modname())

And add "usagehelp", change:

description = S("Wood Pontoon"),_doc_items_longdesc = S("A hollow wooden block designed to be built on the surface of liquids."),

with

description = S("Wood Pontoon"),_doc_items_longdesc = S("A hollow wooden block designed to be built on the surface of liquids."),_doc_items_usagehelp = S("Usage Help."),