Documentation/merge-options.txt: group "ff" related options together

The --ff-only option was not described next to --ff and --no-ff options in
"git merge" documentation, even though these three are logically together,
describing how to choose one of three possibilities.

Also the description for '--ff' and '--no-ff' discussed what '--ff' means,
and mentioned '--no-ff' as if it were a side-note to '--ff'.

Make them into three top-level entries and list them together. This way,
it would be more clear that the user can choose one from these three.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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