Discourage users from using `--merge` in `traverse`, `update` etc.

[PawelLipski]

• [ ] add a disclaimer in docs, with a link to https://github.com/VirtusLab/git-machete#can-i-use-git-merge-for-dealing-with-stacked-prs (I'd keep that link even in CLI docs)
• [ ] maybe display a warning when --merge is passed? or even require a confirmation (to be switched off with some git config advice.<something> false)?