Fix some yarddoc issues

fixes:

```
[warn]: Unknown tag @returns in file `lib/rex/mime/part.rb` near line 41
[warn]: Unknown tag @returns in file `lib/rex/mime/part.rb` near line 41
[warn]: Unknown tag @returns in file `lib/rex/post/meterpreter/extensions/extapi/wmi/wmi.rb` near line 31
[warn]: Unknown tag @returns in file `lib/rex/post/meterpreter/extensions/extapi/adsi/adsi.rb` near line 37
```