create_lua_library lua doc generation needs path controls, opt-out

Not certain if this is the exact best way to do this, but...

A recent update adds lua doc generation. This breaks my local project.

1. Users of create_lua_library library won't *always* want lua docs.

2. The luaDocPath is *HARDCODED*, obviously breaking any attempts to run create_lua_library against anything other than Polycode itself.

Solution: add an optional luaDocPath argument. If omitted, this falls back on using the current hardcoded path as a default. If the argument is equal to "-" then lua docs are not written at all.