I have an older version here, so maybe it's different now, but I assume there's a header with some information, which you should read carefully.  At least in some earlier versions, it is not recommended to edit the acad2xxx.lsp file, but there isn't an acad2xxxdoc.lsp file until some later version than mine, so maybe it's okay to edit that.  If so, yes, you can just add that line anywhere other than inside some other function in it.  If it's not recommended, and you don't have an acaddoc.lsp file already, you can make one, in some folder that's in your Support File Search Path list, even if it contains only that one (autoload) function -- you'll develop other things you want to put in it.  It is run every time a drawing is opened, so things in it are available in all drawings, and maybe that's done with acad2013doc.lsp, too.  The issue is that some related files, like the one you probably have without the doc in its name, may be loaded only once when AutoCAD is started, and not with every drawing that's opened, so commands defined in them won't necessarily be available in all drawings.  So read up on the files and which are loaded when.