The .tagdoc extension uses it's own special syntax for defining language constructs. It's a way for us to create tagging symbols for a variety of languages that do not have a full-blown parser.
However, there is a real parser now for Lua, so you can create tags for any file with a .lua extension. (I forget exactly which version introduced the Lua parser, but it's been a couple years now)
If you open the Tools > Tag Files configuration dialog, you may already see a Lua tag file which has the contents of the lua.tagdoc symbols. You can select that tag file, use the "Add Files…" button and browse to any .lua file you wish to have symbols for. (Yes, you can mix .tagdoc and "real" files in the same tag file)