lposix.c: document posix.wait's return values properly
lposix.c: document posix.wait's return values properly

jimmyharris/luaposix,snegovick/luaposix,luaposix/luaposix,rrthomas/luaposix,ardje/luaposix,snegovick/luaposix,jimmyharris/luaposix,ardje/luaposix,zevv/luaposix,rrthomas/luaposix,zevv/luaposix