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.
Right. It will or can only happen when you do the rebase before you actually pull the latest changes from the upstream repository. In such a case your patch will be merged on top, and a merge commit will remain.