[GSoC][RFC PATCH] git-merge.adoc: detail submodule merge

Submodule merges are, in general, similar to other merges based on oid three-way-merge. When a conflict happens, however, Git has two special cases on handling the conflict before yielding it to the user. From the merge-ort and merge-recursive sources:

- "Case #1: a is contained in b or vice versa": both strategies try to
perform a fast-forward in the submodules if the commit referred by the
conflicted submodule is descendant of another;

- "Case #2: There are one or more merges that contain a and b in the
submodule.  If there is only one, then present it as a suggestion to the
user, but leave it marked unmerged so the user needs to confirm the
resolution."

Add a small paragraph on git-merge.adoc describing this behavior.

Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
---

Hi,

This is a "scratch-my-own-itch" documentation patch. Some years ago I was questioned why some submodule merges on GitHub lead to conflicts while locally they didn't. I only could find a answer for that reading the merge-ort source code, then a wrote a blog post about that, which you check here: https://lucasoshiro.github.io/posts-en/2022-03-12-merge-submodule/

Thus, this patch adds to the official documentation what I found at the time. I wasn't certain if this should belong to the submodule or merge documentation, so, by now, I'm sending it as a merge documentation patch.

 Documentation/git-merge.adoc | 7 +++++++
 1 file changed, 7 insertions(+)