Update docs to force us using rebase and not cherry-pick for merges (#486)

This is the fix for issue #486. It updates the docs for merging branches, by removing the lines for `cherry-pick` actions. Those were actually never needed so far, but always caused us trouble.

@andreieftimie can you please have a look at? Thanks.
This shouldn't usually happen, right? So this step will usually not be run...