v3merge-strategies.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 (introduced in 68d03e4a6e44) 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 merge-strategies.adoc describing this behavior.
>
> Helped-by: Junio C Hamano <gitster@pobox.com>
> Helped-by: Elijah Newren <newren@gmail.com>
> Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
> ---
>
> This v3 keeps the same content as the previous v2. However, it uses the
> proper way to declare paragraphs inside explanations and replaces the
> duplicated paragraph in `recursive` strategy by a small paragraph
> telling that it behaves the same way as `ort` when merging submodules.
>
>  Documentation/merge-strategies.adoc | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/Documentation/merge-strategies.adoc b/Documentation/merge-strategies.adoc
> index 5fc54ec060..9c30f1c900 100644
> --- a/Documentation/merge-strategies.adoc
> +++ b/Documentation/merge-strategies.adoc
> @@ -22,6 +22,13 @@ ort::
>  	was written as a replacement for the previous default
>  	algorithm, `recursive`.
>  +
> +In the case where the path is a submodule, if the submodule commit used on
> +one side of the merge is a descendant of the submodule commit used on the
> +other side of the merge, Git attempts to fast-forward to the
> +descendant. Otherwise, Git will treat this case as a conflict, suggesting
> +as a resolution a submodule commit that is descendant of the conflicting
> +ones, if one exists.
> ++
>  The 'ort' strategy can take the following options:

> @@ -95,6 +102,10 @@ recursive::
>  	renames.  It does not make use of detected copies.  This was
>  	the default strategy for resolving two heads from Git v0.99.9k
>  	until v2.33.0.
> +
> ++
> +For a path that is a submodule, the same caution as 'ort' applies to this
> +strategy.
>  +
>  The 'recursive' strategy takes the same options as 'ort'.  However,
>  there are three additional options that 'ort' ignores (not documented