Add detailed explanation of merge options

I recently had a long discussion with a customer about the difference between all of our project merge options. Specifically, they were under the impression that a merge commit would consolidate the commits from the feature branch into the merge commit. They were confused why they saw both the merge commit and the individual commits from the branch.

We should explain all of this in detail and provide screenshot examples of the history page.

cc/ @axil

So they thought that by merging an MR all commits are squashed in one commit, the one that is created by the MR?

How many merge options do we have? I know fast-forward which is EE-only, is there any other?

mentioned in commit 063e9f9a

Mentioned in commit 063e9f9a

changed milestone to %8.17

mentioned in issue gitlab-com/gitlab-docs#69 (closed)

changed milestone to %9.0

added docs-todo label

All merge options documented except "merge commit with semi-linear history" (including new squash on merge).

added docs-write and removed docs-todo labels

assigned to @SeanPackham

created branch 257-add-detailed-explanation-of-merge-options

mentioned in merge request !1350 (merged)

assigned to @axil

added docs-review and removed docs-write labels

removed assignee

closed via merge request !1350 (merged)

mentioned in commit cabd2e8d