Update docs to refer to correct option lua_omni_blacklist and provide option to inspect omni completion module loading list

Replaced references to non-existent option `lua_omni_module_blacklist` by references to `lua_omni_blacklist` in the documentation.

As I found it rather cumbersome to track down modules having side effects while loading modules for omni completion, I suggest something along the line of 2c69923 which -- at least for me -- was a great help.