Mention 'Reset Empty Clipboard' Command in Wayland FAQ

For some time I've been wondering why I could paste from copyq's tabs but not with ctrl+v. TIL that it is because Wayland clears the clipboard when the owner of the clip exits. For example, this happens every time I take a screenshot, because the screenshot app exits immediately.

I found that a workaround for this is to use the ['Reset Empty Clipboard' Command](https://github.com/hluk/copyq-commands/blob/master/commands/reset-empty-clipboard.ini) and it works just fine. I feel like since this is basically required for Wayland, it would be worth a mention in the [FAQ](https://copyq.readthedocs.io/en/latest/known-issues.html#known-issue-wayland)

Thanks!

Start with the Wayland section of the FAQ at known-issues.html#known-issue-wayland and review the linked Reset Empty Clipboard Command. Add a note explaining why the command is useful on Wayland and link to it; the FAQ should clearly mention this workaround.

Written by the indexing model from the issue text.